Fuses

A fuse is designed to break current flow in the circuit when there’s excessive current flowing through a circuit.

A fuse contains a strip of metal, which when exposed to higher current than designed for will melt/break before any damage can be done to the wires in the circuit.

Example: A circuit with #14 wire is rated for 15A with a suggested 15A fuse. If you have 18A flowing through this circuit, the wire insulation could become damaged and possibly start a fire. The fuse is designed so the metal link inside the fuse will heat up and break the flow of current.

Plug and cartridge are the two main types of fuses.

Plug Fuses

Plug fuses come in sizes 15A, 20A, and 30A. They screw into your electrical panel and are most common in older residential houses.

Type D Plug Fuses: Type D fuses allow for a time delay. Your circuit can withstand a surge of electricity without blowing a fuse right away. Type D fuses would be used in circuits running motors or electrical heat

Type P Plug Fuses: Type P fuses are non-time delay standard fuses. These would be used in standard circuits which contain lights and ordinary household appliances

Type S Plug Fuses: Type S fuses are time-delayed fuses and also tamper-resistant. You cannot use an incorrect fuse as you have to screw them into a base designed for that specific amperage fuse

Cartridge Fuses

Cartridge fuses are the other main type of fuse named in this lesson, with plug fuses. Like a fuse described above, a cartridge fuse is designed to break current flow in the circuit when there is excessive current flowing through the circuit.

The metal link inside the fuse is what breaks the flow. When it is exposed to higher current than designed for, it will melt and break before damage can be done to the wires in the circuit. The protection for a circuit must match the rating of the wire in that circuit, as in the example in this lesson of a circuit with #14 wire rated for 15A with a suggested 15A fuse.

Circuit Breakers

A circuit breaker serves the same basic purpose as a fuse. It is designed to break current flow in the circuit when there is excessive current flowing through the circuit. Like a fuse, it is there to act before damage can be done to the wires in the circuit.

The example given for fuses shows why the rating matters. A circuit with #14 wire is rated for 15A, with a suggested 15A fuse. If 18A flows through this circuit, the wire insulation could become damaged and possibly start a fire. The protection for a circuit must match the rating of the wire in that circuit.

Circuit breakers and fuses will protect against both overloaded circuits as well as overcurrent in a circuit. It’s important to know the difference between both.

Overload

An overload in a circuit is when there’s an increase of current (amperage) above the rating for the circuit. If the circuit is allowed to draw the excess current, the wire insulation could become damaged or even catch fire from the added heat. An overload in a circuit usually happens over time and not instantly.

A circuit with #14 wire is rated for 15A with a 15A breaker required. If you use a 20A breaker instead, you could run into problems if amperage in the circuit goes above 15A as the wire is not designed to handle that much. It will overheat and could become damaged.

A seized blower motor attempting to start and run could add to an overload condition. but probably not create an overload condition as most motors in the HVAC industry have built-in thermal overload protection to shut the motor down when it begins to overheat.

Too many pieces of equipment on a circuit could create an overload condition.

Overcurrent

Overcurrent situations usually happen almost instantly. Overcurrent could be a short circuit or a sudden rush of current upon a motor startup in a circuit not designed to handle it.

Upon motor startup, a motor will draw a higher amount of amperage than it will during normal operation. This is called the LRA, lock rotor amperage. It’s the amperage a motor will draw when the supply voltage is applied, and the rotor is locked (not turning). This lasts for a second or less before the motor gets up to speed.

Overcurrent protection happens much quicker than overload protection.

So let’s quickly recap the representations for voltage, resistance, and amperage in the Ohm’s Law triangle:

E = Voltage (electromagnetic force)

I = Current (amps)

R = Resistance

For a given resistance, the current is directly proportional to voltage. That said, if you increase the voltage in a circuit with a fixed resistance, the current will increase. If you decrease the voltage in a circuit with a fixed resistance, the current will decrease.

Ohm’s law follows a simple formula:

E = I x R

To quickly recap again; E is voltage, I is current (amps), and R resistance (ohms).

Using Ohm’s Law allows you to find an unknown voltage, current, or resistance as long as you have at least two values of the triangle.

Now, I said you could solve for voltage, current or resistance as long as you have at least two values, yet the formula above only allows for voltage (E) to be solved. Ohm’s Law triangle can be rearranged for different values to be solved.

Solving for unknown voltage: E = I x R

Solving for unknown current: I = E / R

Solving for unknown resistance: R = E / I

When using the triangle, just cover the unknown value with your finger and solve it. Imagine a division symbol under E and a multiplication symbol between I and R.

Example 1: You’re working on a circuit with an unknown voltage. We know the current is 0.5A and the resistance is 10Ω. What’s the voltage?

Keep in mind; we only cover the unknown value we want to solve for. So, we would cover the E in the triangle since we’re trying to determine the voltage. We know we will be using multiplication since we read earlier to imagine a multiplication symbol between I and R.

E = I x R

E = 0.5A x 10Ω

E = 5V

Example 2: You’re working on a circuit with an unknown current. We know the voltage is 5V and the resistance is 10Ω. What’s the current?

Just like earlier, we’re going to cover the symbol we’d like to solve for. So, we will cover the I in the triangle now. We know we will be using division since we read earlier to imagine a division symbol below E

I = E / R

I = 5V / 10Ω

I = 0.5A

Example 3: You’re working on a circuit with unknown resistance. We know the voltage is 5V and the current is 0.5A. What’s the resistance?

Nothing has changed in how we are going to solve this. Just like before, we are going to cover the symbol we’re want to find. So, we would cover the R in the triangle since we need to solve for resistance. We know we will be using division since we read earlier to imagine a division symbol below E

R = E / I

R = 5V / 0.5A

R = 10Ω

What this means on a service call

Ohm’s Law is the check that ties a reading to a decision. If you know two of the three values, voltage, current, and resistance, the third value must agree. Use it before you replace a part. A reading that does not agree with the other two usually points to a test problem, a wrong test point, or another part in the path, not to a failed load by itself.

On a service call, measure the voltage across the load and the current through it, then work out the resistance and compare it with the power-off resistance check. If the numbers do not line up, retest before you condemn the part. Record the test points with each reading.

Numbers and measurements you will see

The numbers below use the examples in this post and the series and parallel checks. All three forms must agree for the same circuit.

• Voltage from current and resistance. E equals I times R. In the example, 0.5A times 10 ohms gives 5V.
• Current from voltage and resistance. I equals E divided by R. In the example, 5V divided by 10 ohms gives 0.5A.
• Resistance from voltage and current. R equals E divided by I. In the example, 5V divided by 0.5A gives 10 ohms.
• Series check. In the series example, 12V divided by 10 ohms gives 1.2A, and 1.2A times 10 ohms gives 12V. The same current, total resistance, and total voltage must agree.
• Parallel check. In the parallel example, 12V divided by 8 ohms gives 1.5A total. The branch currents, 1A and 0.5A, add to 1.5A.
• Direction of change. For a fixed resistance, current follows voltage. For a fixed voltage, current falls when resistance rises.

A common mistake

The most common mistake is to use the wrong two values in the formula. The voltage, current, and resistance must belong to the same part of the circuit, measured at the same test points. Do not use the applied voltage for the whole circuit when you want the drop across one part in series, and do not use the total resistance when you want the current in one parallel branch. Pick the part, pick the test points, then use the values for that part.

A second mistake is to trust one calculated value and stop. If the calculated resistance does not fit what the resistance check shows, or the calculated current does not fit what the meter shows, something is wrong with a test, a connection, or the circuit path. Retest the two known values first. Do not replace a load on one calculation alone.

Check it in the field

1. Record two measured values and the test points. Note whether the circuit was live for a voltage or current check, or off for a resistance check.
2. Use the correct form for the unknown value. Cover the unknown in the triangle, then write the formula, the numbers, and the answer before you act on it.
3. Compare the calculated value with a direct measurement where you can make one. In the example, a calculated 10 ohms should agree with the resistance check, and a calculated 0.5A should agree with the current check.
4. Cross-check with the circuit rules. In series, check that the current is the same and the voltage drops add to the applied voltage. In parallel, check that the voltage is the same and the branch currents add to the total.

Make resistance checks with the power off, and prove the circuit dead with a voltage test first. Keep hands clear of bare parts during live voltage and current checks. If a reading does not agree after a retest, stop and check the meter leads, the range, and the test points before you change a part.

Related posts

These posts belong together.

• Voltage, current, and resistance summary: a short recap of voltage, current, and resistance together.
• Voltage: the push that makes current flow, and how to read a voltage test.
• Current (amps): what flow means, and how to check it without guessing.
• Resistance: what opposition to flow means, and why a load must have resistance.
• Series Circuits: one path for current, with the same current through each part.
• Parallel Circuits: more than one path for current, with the same voltage across each branch.

A parallel circuit allows for more than one path for current to flow. We have the same two loads from the series circuit below, but we have two paths now for electrons to flow. Each pathway for current to flow is called a branch.

2 loads in series Vs. 2 loads in a parallel circuit

In a parallel circuit, if the light (load) in branch 1 were to be faulty, the light (load) in branch two would still function as there’s another pathway around the faulty load to take for the electrons to flow.

Rules of a Parallel Circuit

• Rule 1: The voltage across each branch of a parallel circuit will be the same as the applied voltage.

V total = V1 = V2 = V3 = …

• Rule 2: The total current of a parallel circuit is the sum of the current in each branch.

I total = I1 + I2 + ….

I total = 1A + 1A

I total = 2A

We have a total of 2A split between 2 different loads in this circuit.

The amount of current that travels through each branch will not necessarily be the same because the resistance of the load in each branch will dictate the current flow. The higher the resistance in the load, the less current flow

• Rule 3: The total resistance in a parallel circuit is less than any of the individual resistances.

Calculating Total Resistance With 2 Resistors:

Calculating resistance total with only two resistors in parallel is a bit easier than calculating for 3 or more.

Example:

Resistor 1: 12Ω

Resistor 2: 24Ω

R total = R1 x R2 / R1 + R2

R total = 12Ω x 24Ω / 12Ω + 24Ω

R total = 288Ω / 36Ω

R total = 8Ω

Calculating Total Resistance with 3 or More Resistors:

The formula for calculating 3 or more resistors is a little different and there’s a bit more to it.

Example:

Resistor 1 = 5Ω

Resistor 2 = 20Ω

Resistor 3 = 10Ω

Resistor 4 = 1Ω

R total = 1/ (1/R1 + 1/R2 + 1/R3 + 1/R4)

R total = 1/ (1/5Ω + 1/20Ω + 1/10Ω + 1/1Ω)

R total = 1/ (.20Ω + .05Ω + .1Ω + 1Ω)

R total = 1/1.35Ω

R total = .74Ω

Calculating Current in Each Branch

Now that we know the total resistance of this circuit to be 8Ω, calculating current in each branch comes down to basic Ohm’s Law.

Step 1: We need to calculate the current for the whole circuit.

I = E / R

I = 12V / 8Ω

I = 1.5A

The total current through the circuit is 1.5V. We know that each branch will have different current since the resistive loads are different sizes.

Step 2: Calculate the current for each branch separately.

Resistor 1: 12Ω

I = E / R

I = 12V / 12Ω

I = 1A

Resistor 2: 24Ω

I = E / R

I = 12V / 12Ω

I = .5A

Step 3: Confirm individual current calculations.

We can confirm our individual branch calculations are correct by adding them together and comparing them to our total circuit current calculation.

I total = I1 + I2 + ….

I total = 1A + .5A

I total = 1.5A

1.5A is exactly what we calculated in step 1.

A Broken Path Will Not Stop Current Flow

If a path for current to flow is broken, current will continue to flow in other paths that are still available.

What this means on a service call

A parallel circuit gives each load its own path, and the voltage is the same across each branch. That changes what a failure means. If one branch opens, the other branches can still work. The light in branch 1 can fail while the light in branch 2 still runs, because current still has a path around the faulty load.

On a service call, that independence changes the diagnosis. A dead load does not prove that power is gone. Check that branch first. A working load proves voltage is present at the common points, but it does not prove every branch is good. Test each branch on its own before you call the circuit good.

Numbers and measurements you will see

The numbers below all come from the 12V example with a 12 ohm resistor and a 24 ohm resistor. Use the three rules to check each other. Voltage stays the same, currents add, and total resistance falls below any one branch.

• Voltage is the same, 12V. Each branch sees the full applied voltage. Do not split the voltage between branches as you would in series.
• Total resistance is 8 ohms. In the example, 12 ohms and 24 ohms in parallel give 8 ohms total. That total is lower than either branch alone.
• Total current is 1.5A. In the example, 12V divided by 8 ohms gives 1.5A for the whole circuit.
• Branch 1 current is 1A. In the example, 12V divided by 12 ohms gives 1A.
• Branch 2 current is 0.5A. In the example, 12V divided by 24 ohms gives 0.5A. The higher resistance branch carries less current.
• Check by adding branches. In the example, 1A plus 0.5A gives 1.5A, which matches the total current. In the first example in this post, 1A plus 1A gives 2A total.

A common mistake

The most common mistake is to use series thinking in a parallel circuit. Do not expect the current to be the same in each branch, do not add the resistances to get the total, and do not split the voltage between branches. In parallel, voltage is the same, resistance total falls, and branch currents differ when branch resistances differ.

A second mistake is to measure the total current and assume each branch carries that much. In the example, the total is 1.5A, but no branch carries 1.5A. Branch 1 carries 1A and branch 2 carries 0.5A. Measure each branch on its own, and add the branch currents to check the total.

Check it in the field

1. Check the applied voltage across the branches. Each branch should show the same voltage. Record the voltage and the test points.
2. Measure current in each branch separately. Clamp one insulated conductor for one branch only. Keep hands clear of bare parts, and record each branch reading.
3. Add the branch currents together and compare the sum with the total current. In the example, the branch currents must add to 1.5A. If they do not, recheck the test points and the meter.
4. If a branch shows 0A while voltage is present across it, check that branch only. With the power off, and the circuit proved dead, check continuity through the load and wiring in that branch until you find the break.

Do resistance and continuity checks with the power off. Prove the circuit dead with a voltage test first. Do not clamp more than one conductor at once when you want a branch reading. Stop and get help if you are unsure about the power state.

Related posts

These posts belong together.

• Voltage, current, and resistance summary: a short recap of voltage, current, and resistance together.
• Voltage: the push that makes current flow, and how to read a voltage test.
• Current (amps): what flow means, and how to check it without guessing.
• Resistance: what opposition to flow means, and why a load must have resistance.
• Series Circuits: one path for current, with the same current through each part.
• Ohm’s Law: how voltage, current, and resistance relate, with worked examples.

series

noun

1. a number of things, events, or people of a similar kind or related coming one after another.

So given the above definition, a series circuit is when there’s only a single path for current to flow. Any break in the wiring or a faulty load will stop current throughout the entire series circuit.

A load is anything in the circuit that does useful work, like the lightbulb in the example below. The inexpensive and/or old Christmas lights have this downfall. When a single bulb burns out, the entire string of lights goes out.

Rules of a Series Circuit

• Rule 1: Current is the same throughout the circuit

Since current stays the same through the circuit, this makes calculations using Ohm’s Law easier.

Each component in a series circuit has the same current flow.

I total = I 1 = I

• Rule 2: The total resistance of a series circuit is equal to the sum of the individual resistances.

Adding the individual value of each component in a series circuit will give the total resistance of the circuit.

In the above example, we have two values.

R total = R1 + R2

R total = 5 Ω + 5 Ω

R total = 10 Ω

• Rule 3: The total voltage drop in a series circuit is equal to the applied voltage of the circuit.

The total voltage drop across each resistor must add up to the total voltage supplied to the circuit from the battery.

V total = V 1 + V 2

V total = 6 V + 6 V

V total = 12 V

Using Ohm’s Law any of these totals can be found as long as you have at least 2 values.

Voltage

E = I x R

E = 1.2 A x 10 Ω

E = 12 V

Resistance

R = E / I

R = 12 V / 1.2 A

R = 10 Ω

Amperage

I = E / R

I = 12 V / 10 Ω

I = 1.2 A

If at any point there’s a break in the wiring or a load is faulty, no current will flow. No flow = no go.

What this means on a service call

A series circuit gives current one path and no way around a break. That is the point to carry into a service call. If one part in the path opens, current stops in the whole path. The Christmas lights in the original post show it plainly. One bulb burns out, the whole string goes out, even though the other bulbs are still good.

Think of switches and safeties in the same way. A switch, a fuse, or a safety control in series with a load can stop the load when it opens. That does not mean the load failed. It means the path is open somewhere. Find the open part before you replace a load that may still be good. Check each part in the path in turn, and do not assume the first part you test is the fault.

Numbers and measurements you will see

The numbers in this post all come from one example, and they must agree with each other. Use that agreement as a check. If a reading does not fit the three rules, the circuit in front of you is not the example, or a part has changed.

• Current is the same, 1.2A. In the example, 12V divided by 10 ohms gives 1.2A. That same 1.2A flows through each part. A different current at two points in one series path means a reading error, a second path, or a changed circuit.
• Resistance adds, 10 ohms. In the example, 5 ohms plus 5 ohms gives 10 ohms total. Add each part to get the total. The total is higher than any one part.
• Voltage drops add, 12V. In the example, 6V plus 6V gives 12V total. The drops across the parts must add up to the applied voltage. If the drops do not add up, a reading is wrong or a part is missing from the count.
• Ohm’s Law check for voltage. E equals 1.2A times 10 ohms, which equals 12V.
• Ohm’s Law check for resistance. R equals 12V divided by 1.2A, which equals 10 ohms.
• Ohm’s Law check for current. I equals 12V divided by 10 ohms, which equals 1.2A.

A common mistake

The most common mistake is to replace the load when the path is open somewhere else. A load with 0V across it and no current may be good, with an open switch, fuse, or wire upstream. Test for voltage at the load, then test each part in the path, before you condemn the load.

A second mistake is to expect the full applied voltage across each part. In series, voltage is shared. In the example, each resistor drops 6V, not 12V. A 6V reading across one part can be correct. Add the drops together before you call a reading wrong. A third mistake is to measure current at one point and assume it will differ at another point in the same series path. It will not, unless the path has changed.

Check it in the field

1. Check the applied voltage at the source, with the circuit powered as the equipment instructions allow. Record it. In the example, that value is 12V.
2. Measure the voltage drop across each part in the path, one part at a time. Record each drop. Add the drops together and compare the total with the applied voltage.
3. Check current in the path. In a series path, the current should be the same at each test point. Use Ohm’s Law to cross-check the current, resistance, and voltage figures against each other.
4. If current is 0A, find the open. With the power off, and the circuit proved dead, check continuity through each part and each wire in the path, one at a time, until you find the break. Repair only the open part, then retest.

Do resistance and continuity checks with the power off. Prove the circuit dead with a voltage test first. Do not touch bare metal while you test a live circuit. Keep hands clear, use insulated leads, and stop and get help if you are unsure about the power state.

Related posts

These posts belong together. Use the summary for the short version, then work through each part on its own.

• Voltage, current, and resistance summary: a short recap of voltage, current, and resistance together.
• Voltage: the push that makes current flow, and how to read a voltage test.
• Current (amps): what flow means, and how to check it without guessing.
• Resistance: what opposition to flow means, and why a load must have resistance.
• Parallel Circuits: more than one path, with total resistance lower than any one branch.
• Ohm’s Law: how voltage, current, and resistance relate, with worked examples.

To quickly sum up voltage, current, and resistance.

Voltage is the driving pressure which is the high and low potential in the water tower below.

Current (amperes or amps) is the resulting flow which is the water flowing through the pipe in the water tower below.

Resistance completes useful work. The wheel spinning in the water tower below.

Read each lesson in full

Use the summary above to keep the three ideas straight, then read each lesson below for the full explanation. Start with voltage, current, and resistance, then read how they work together in series and parallel circuits and in Ohm’s Law.

• Voltage
• Current (amps)
• Resistance
• Series Circuits
• Parallel Circuits
• Ohm’s Law

Almost all material has some form of resistance to electrical flow. Without resistance, you’d have no way for a circuit to do useful work. Like stated earlier, conductors are materials that allow current to flow, and insulators don’t. Conductors have little resistance.

Resistance in a circuit is measured in Ohms or the Greek letter “Ω.” Current flowing through an electric range or toaster causes the elements to generate heat to cook your food by using resistance. In Ohm’s Law, resistance is represented by “R.”

All good loads, like a motor or light, will have some form of measurable resistance otherwise current would easily pass through the load without completing any useful work like lighting up the room or moving air throughout your house.

Example: Water flowing through a garden hose without a kink has less resistance and allows water to flow at a faster rate

What this means on a service call

Resistance is what turns current into useful work. A good load has resistance you can measure. A motor, a heater, or a light does work because current meets resistance in that load. Without resistance in the load, current would pass through without doing the work you need.

Resistance also shows up where you do not want it. A loose or corroded connection adds resistance in series with the load. That added resistance makes heat, causes voltage drop, and starves the load. The load may run weak, the connection may run hot, or the load may not run at all. Check the connection before you condemn the load.

Numbers and measurements you will see

Do not use a single ohm value as a pass mark for every part. This site does not give a resistance rating for each motor, heater, or control in the cluster posts. Compare a reading with the diagram, the nameplate, or the manual for the unit in front of you. The examples below come from posts on this site, and they show how resistance fits with voltage and current.

• 10 ohms in Ohm’s Law. In the Ohm’s Law example, 5V divided by 0.5A gives 10 ohms. If you know voltage and current, you can check whether a resistance value makes sense.
• 10 ohms in series. In the series example, 5 ohms plus 5 ohms gives 10 ohms total. Resistance adds in series, so the total is higher than any one part.
• 8 ohms in parallel. In the parallel example, 12 ohms and 24 ohms in parallel give 8 ohms total. Resistance in parallel is lower than any one branch.
• OL, or open. Most meters show OL when the path is open. That means no complete path for current between the leads. An open winding, a broken wire, or an open switch can all read OL. Check the test points before you condemn the part.
• Near 0 ohms. Touching the leads together should read near 0 ohms. A closed switch, a good wire, or a good connection should also read near 0 ohms. Near 0 ohms across a load that should have resistance can mean a short, so check what the part should read.
• Higher than expected. A reading much higher than the value for that part can mean a poor connection, a damaged winding, or a part that is failing. Repeat the test with clean, firm lead contact, and isolate the part before you trust the number.

A common mistake

The most common mistake is to measure resistance on a live circuit. A meter in ohms mode sends its own small test current through the part. Voltage from the circuit will confuse the meter, give a false reading, and can damage the meter. Prove the circuit is dead first, with a voltage test you trust, then switch to ohms. If you are not sure the circuit is dead, do not take an ohms reading. Stop and prove it dead first.

A second mistake is to measure a part while it is still connected to the rest of the circuit. Other branches give current another path, so the meter can read lower than the part alone. The parallel rules explain why. Disconnect or isolate at least one side of the part, where the diagram and safe practice allow it, so the meter sees only the part you want to test. A third mistake is to condemn a part on one reading alone. Compare the same type of winding or the same type of part, check the leads, and repeat the test. One number, without context, is not a diagnosis.

Check it in the field

1. Turn the power off for the circuit you will test. Prove it dead with a voltage test, using a meter you have checked on a known live source. Follow the equipment instructions if a component can hold a charge.
2. Isolate the part. Disconnect at least one side, where safe practice allows it, so other branches do not give a parallel path.
3. Set the meter to ohms. Touch the leads together and note the reading. It should be near 0 ohms. Hold the leads apart and note the reading. It should show OL. If either check fails, fix the meter or the leads first.
4. Put one lead on each end of the part, with clean and firm contact. Record the reading, the meter range, and the test points. Compare it with the diagram, the nameplate, or the manual.

Do not touch the metal tips of the leads while you take a reading. Your body can give current another path and change the result. Keep hands clear, hold the insulated parts of the leads, and let the reading settle before you record it. If you are unsure about the power state, stop and get help.

Related posts

These posts belong together. Use the summary for the short version, then work through each part on its own.

• Voltage, current, and resistance summary: a short recap of voltage, current, and resistance together.
• Voltage: the push that makes current flow, and how to read a voltage test.
• Current (amps): what flow means, and how to check it without guessing.
• Series Circuits: one path for current, with total resistance as the sum of the parts.
• Parallel Circuits: more than one path, with total resistance lower than any one branch.
• Ohm’s Law: how voltage, current, and resistance relate, with worked examples.

Sticking with the water analogies, amperes, or amps for short, is the amount of water flowing through the hose. The higher the pressure (voltage), the more water flows (amps) out of the hose. In Ohm’s Law, amperage is represented by “I.”

What this means on a service call

Current is the work happening. Voltage can be present at a load, and still no work is done, if no current flows. On a service call, ask the current question early. Is the load drawing current, is it drawing none, or is it drawing more than it should? The answer points you in different directions. No current points to an open path or a load that never started. Too much current points to a load that is working too hard, a short, or a fault that is forcing extra flow through the circuit.

Use the circuit layout to read a current value. In a series circuit, the same current flows through each part. The series example on this site uses 1.2A through a 10 ohm total, with 12V applied. In a parallel circuit, the total current is the sum of the branch currents. The parallel example uses 1A plus 1A for 2A, and later 1A plus 0.5A for 1.5A. If you clamp the main feed, you see the total. If you clamp one branch, you see only that branch. Both readings are useful, but they answer different questions. Write down which conductor you clamped.

A control signal and a load current are not the same job. A control circuit, often 24V, can send a signal with a small current. The line circuit, often 120V, carries the current that runs a motor, a fan, or a heater. Do not judge a motor by a control reading, and do not judge a control by a motor reading. Check the control first to see if the call arrived. Then check the line current to see if the load answered. If the control is present and the line current is 0A, the break is in the line path or in the load itself.

Numbers and measurements you will see

Compare every reading with the wiring diagram, the nameplate, or the manual for the unit in front of you. The examples below come from posts on this site. They show how readings fit together. They are not a rating for every motor or heater.

• 0A. No current is flowing in that conductor. The load may be off, a switch may be open, or the path may be broken. Check voltage upstream before you assume the load failed.
• 1.2A in series. In the series example, 12V divided by 10 ohms gives 1.2A. That same 1.2A flows through each part in the series path. A different reading in a series path means the circuit is not the one in the example, or a part of it has changed.
• 2A in parallel. In the first parallel example, 1A plus 1A gives 2A total. The total is the sum of the branches, even when the branches are equal.
• 1.5A in parallel. In the worked parallel example, 1A plus 0.5A gives 1.5A total. The branches do not have to carry the same current. Resistance in each branch sets its share.
• 0.5A from Ohm’s Law. In the Ohm’s Law example, 5V divided by 10 ohms gives 0.5A. Use that same method to check whether a reading makes sense for the voltage and resistance you have.
• 15A and 18A fuse example. On the circuit breakers and fuses post, a circuit with number 14 wire is rated for 15A with a 15A fuse, and 18A through that circuit is excessive. Excessive current is the reason a fuse or breaker must break the flow before the wire is damaged.

A common mistake

The most common mistake is to clamp more than one conductor at once. A clamp meter reads the field around one conductor. If you clamp a hot and a neutral together, the fields can cancel, and the meter can read 0A even though current is flowing. Clamp one insulated conductor only. Keep the clamp jaws closed, keep the conductor centred in the jaws as far as the meter allows, and take the reading again if the first result does not match the load state.

A second mistake is to break a live circuit to insert a meter in series when a clamp reading would do the job. Breaking a circuit under load can arc, and it puts the meter and the tech in the path of the current. Use a clamp meter around one conductor for a running current check. Save a series meter connection for a test you have planned, with the power state and the meter rating checked first. A third mistake is to read current and stop there. Current alone does not prove voltage is correct, and it does not prove resistance is correct. Use all three together, as in Ohm’s Law, before you change a part.

Check it in the field

1. Find the conductor for the load you want to check. Set the clamp meter for the right kind of current and a range that covers the circuit. Check that the jaws are clean and close fully.
2. Clamp one insulated conductor only. Keep your hands clear of bare parts. Record the reading, the conductor, and whether the load was off, starting, or running.
3. Compare the reading with the nameplate, the diagram, or the manual. If no value is given there, compare the reading with the same load under the same conditions, and record both readings.
4. If the reading is 0A and the load should run, check voltage at the load next. If voltage is present and current is 0A, the load or its internal path is open. If voltage is missing, work upstream.

Work with care around live conductors. Do not touch bare metal to get a better clamp position. Do not open a live circuit to insert a meter unless that test is planned and the meter is rated for it. If a reading is far above what the diagram or nameplate shows, stop the load if it is safe to do so, and find the cause before you run it again. If you are unsure, stop and get help.

Related posts

These posts belong together. Use the summary for the short version, then work through each part on its own.

• Voltage, current, and resistance summary: a short recap of voltage, current, and resistance together.
• Voltage: the push that makes current flow, and how to read a voltage test.
• Resistance: what opposition to flow means, and why a load must have resistance.
• Series Circuits: one path for current, with the same current through each part.
• Parallel Circuits: more than one path, with total current as the sum of the branches.
• Ohm’s Law: how voltage, current, and resistance relate, with worked examples.

Earlier we covered electrons moving from atom to atom. Metallic bonding forms copper. The bond is made when many metal atoms share their collected valence electrons. The valence electrons are not attached to any specific atom, so they move around freely.

Now to get these electrons moving in one direction and being of more use, they just need a push. This is by applying an electromotive force (emf), also known as voltage.

Voltage can be compared to the pressure water is forced out of your kitchen faucet. It would be the water pressure that caused the water to flow.

When you measure voltage, you measure the potential difference between two points in a circuit. It’s important to remember this because you may measure 0 volts and take a good shock if you touch that wire.

In Ohm’s Law, voltage is represented by “E.”

What this means on a service call

Voltage tells you whether the push is present at the point you are testing. It does not tell you whether a load is good. It does not tell you how much current is flowing. On a service call, start with that simple question. Is voltage reaching the part that should run, or is it stopping at a switch, a control, or a broken wire before it? Do not jump to the load until you know the push reached it.

Keep the control side and the line side separate in your mind. The control circuit, often 24V, tells the system what to do. The line circuit, often 120V, supplies the power for a motor, a fan, or a heater. A system can have control voltage and still have no line voltage at a load. It can also have line voltage present and still not run because the control signal never arrived. Test each side on its own. Record the two points you used and the reading.

A dead load with voltage upstream is a different fault from a dead load with no voltage upstream. The first points to the load or the path after your last good reading. The second points to the supply, a control, or a wire before it. Say which case you have before you change a part. That one sentence, written on the job notes, will save a second visit.

Numbers and measurements you will see

Compare every reading with the wiring diagram, the nameplate, or the manual for the unit in front of you. The examples below show what to look for. They are not a pass mark for every unit.

• 24V control. Expect 24V between the control terminals when the control should be on. If you read 0V there, the signal is missing, a switch is open, or a wire is broken upstream.
• 120V line. Expect 120V at the line side of a disconnect or at a load that should be running. If it is present at the input and missing at the output, the break is between those points.
• 0 volts. A 0 volt reading can mean no power, an open switch, or two points at the same potential. It does not prove a circuit is safe to touch.
• Voltage across a switch. Full applied voltage across an open switch can be normal. The push is reaching that point and stopping there. Full voltage across a closed switch points to a bad connection or an open contact.
• Flame sensor example. The 80 to 100V range already used on this site for a flame sensor belongs to that one test. Do not apply that range to other parts.
• Series check. In the series example on this site, 6V plus 6V adds to 12V. In a series circuit, the drops across the parts should add up to the applied voltage.

A load that is running should have voltage across it. A wire or a closed switch should have little voltage across it. If a wire shows a large drop, check the connection at each end. Do not keep running a circuit with a hot connection. Find the cause first.

A common mistake

The most common mistake is to read 0 volts, call the circuit dead, and touch a wire. The original post warns about that exact error. A meter only reports the difference between its two leads. If both leads touch points at the same potential, the meter reads 0 volts even though both points are live to ground. A loose lead, a painted surface, or a corroded terminal can also give a false 0. Seat both leads, clean the test point if needed, and watch the reading while you hold them steady.

A second mistake is to replace a part because voltage is present at its terminals. Voltage present means the push arrived. It does not prove the part can carry current or do work. Check current and resistance, where the test is safe and the power state is right for that test, before you condemn the part.

Check it in the field

1. Set the meter for the right kind of voltage and a range that covers the circuit. Check the leads for damage. Test the meter on a known live source, test the circuit, then test the meter again. If it does not read as expected on the known source, stop.
2. Pick two test points from the diagram. Put one lead on each point, keep your hands clear of bare metal, and record the reading and the points.
3. Move one lead at a time along the circuit. When the reading changes, you have crossed the point where voltage is lost. That narrows the fault to one section.
4. Do not bypass a safety control to get a reading. If a control is open, find why it is open.

Voltage testing is a live test. Treat every conductor as live until you have proved otherwise with a meter you trust. Keep leads and hands away from other bare parts. Use a meter rated for the circuit. If you are unsure, stop and get help.

Related posts

These posts belong together. Use the summary for the short version, then work through each part on its own.

• Voltage, current, and resistance summary: a short recap of voltage, current, and resistance together.
• Current (amps): what flow means, and how to check it without guessing.
• Resistance: what opposition to flow means, and why a load must have resistance.
• Series Circuits: one path for current, and how voltage drops add up.
• Parallel Circuits: more than one path, and why one branch can fail while others keep working.
• Ohm’s Law: how voltage, current, and resistance relate, with worked examples.

A most basic electrical circuit will have three components

• A source of energy. It could be a battery or an electrical outlet in the wall
• A load. A load does useful work. A load could be a light bulb or a motor
• Conductors. A conductor would be your wires to transfer the electrical energy to your load

While not entirely needed, most circuits will include a switch. A switch will stop the flow of electricity through the conductor to the load.

As you can see, with an open switch, power cannot get past the switch to turn on the lights.

With the switch in the closed position, power is allowed to flow through the circuit and turn the lights on.

What this means on a service call

If the load does not run, one part of the circuit is not doing its job. The source, the conductors, the switch, or the load itself has failed, is open, or is not connected.

Start with the path. Power leaves the source, travels through a conductor, passes through a switch or a control, reaches the load, and returns to the source through a conductor. A break anywhere on that path stops the load. A light bulb stays dark and a motor stays still when the path is open, even if the source is good.

On a furnace or air conditioner call, the switch in this lesson may be a thermostat, a limit, a pressure switch, or a relay contact. The load may be a blower motor, an inducer motor, a contactor coil, or a gas valve. Do not guess which part failed. Follow the path in order, from the source toward the load, and find where power stops.

A closed switch does not prove the whole circuit is good. It only proves that one switch is closed. A loose wire, a broken conductor, or a failed source can still leave the load dead. Check each part in turn, and write down what you find at each step so you do not recheck the same part twice.

Numbers and measurements you will see

Use the voltages that belong to the circuit you are working on. A control circuit on HVAC equipment is commonly 24V. Line power to the equipment is commonly 120V. Check the rating on the equipment and on the part before you test, and use the meter setting that fits that circuit.

• Source voltage at the source. With the circuit powered, the meter should read the source voltage, for example 24V on a control circuit or 120V on a line circuit.
• 0V at the load when the switch is open. An open switch stops power from reaching the load, so the load sees no voltage.
• Source voltage at the load when the switch is closed and the path is complete. If the load still does not run at full source voltage, the load itself is suspect.
• Source voltage across an open switch. A meter across an open switch in a powered circuit will often read source voltage, because the open switch is the break in the path.
• Continuity with the power off. A good conductor and a closed switch should show continuity. An open switch, a broken wire, or a loose connection should not.
• Current only when the circuit is complete. Current flows when the switch is closed and the path is complete. No current flows through an open switch, even if voltage is present at the source.

Read the meter at the part you are testing, not only at the equipment disconnect. A reading at the source tells you about the source. A reading at the load tells you whether power reached the load.

A common mistake

Replacing the load first is the common mistake. A technician replaces a motor or a bulb, the new part also does not run, and the real fault is still in the circuit.

Check the source, the conductors, and the switch before you condemn the load. A motor that never gets power cannot run, and a new motor will not fix a broken wire or an open switch. Test for voltage at the load. If voltage never arrives, move back toward the source until you find where it stops. If full source voltage arrives and the load does not run, then look hard at the load.

A second mistake is testing only one point and stopping. One good reading at the source does not prove the conductors are good. One reading at the load does not prove the switch is good. Test the source, test each side of the switch, and test at the load, so each part is proved in turn.

Check it in the field

Caution: live and powered work can hurt you. Treat a circuit as live until you prove it is dead with your meter. Keep your hands clear of bare conductors and terminals. If you are not sure a test is safe, stop, turn the power off, and get help. Do not work on a live circuit with wet hands, and do not bypass a switch or a safety control to force a load to run.

1. Turn the power off before a continuity or resistance check. Continuity and resistance checks are done with the power off.
2. Prove the meter works on a known live source, then prove the circuit is dead, then check continuity through the conductors and through the switch in its closed position.
3. Restore power only for a voltage check, and only if you are trained to work on that circuit. Measure source voltage at the source first.
4. Measure voltage on each side of the switch. With the switch open, expect no voltage at the load side. With the switch closed, expect source voltage to pass through to the load side.
5. Measure voltage at the load. If source voltage is present and the load does not run, the load needs further testing. If source voltage is not present, move back toward the source and find the open part.
6. Turn the power off again before you repair a wire, tighten a connection, or replace a part. Recheck after the repair, and confirm the load runs when the switch is closed and stops when the switch is open.

Related posts

• Polarity
• Direct current
• Alternating current
• Voltage
• Current (amps)
• Resistance
• Series Circuits
• Parallel Circuits
• Ohm’s Law