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M & R ELECTRICAL WHOLESALING

Knowledgebase Article

Fault Loop Impedance

 

Introduction

The fault loop impedance is the most important aspect to take into consideration when designing an electrical circuit. It directly affects the size of the circuit breaker and the size of the cable that can be used on an installation. All electricians have heard of it. However, the author's experience is that almost half of all electricians do not fully understand it. Designing and wiring electrical circuits without a full and comprehensive understanding of this concept can pose a serious safety hazard to the end user. This document will explain in detail the concept of fault loop impedance, how to calculate it and how to ensure your circuits are well designed.

 

A quick note on impedance

In this document the terms impedance and resistance will be used interchangeably. Although technically there is a difference between the two, for this particular application they can be considered the same thing.

 

What exactly is "Fault Loop Impedance"?

The fault loop impedance is exactly what its name implies - it is the impedance (or resistance) of the loop (or circuit) under fault conditions.

 

A fault condition is usually considered a short circuit at the end of the cable run or at the terminals of the load.

 

The "loop" in this case refers to the entire path that the current will flow in the circuit under fault conditions. There are 2 possibilities for a short circuit at the end of the cable run. The first is a phase to phase short circuit. The second is a phase to earth short circuit.

 

The impedance of the phase to earth short circuit is used in these calculations. This is because the earth wire is usually smaller than the active wires, so it will have more resistance than a phase to phase fault. Why we want the higher resistance value will make more sense later.

 

This diagram below shows the current path that would occur under a phase to earth short circuit.

 

 

Why is Fault Loop Impedance important?

The impedance on the circuit determines how much current will flow if there is a short circuit. The cable path effectively acts like a resistor that has been placed across the circuit breaker, as shown in the diagram below.

 

 

When the impedance is high, less current will flow. If it is too high, there is a very real chance that the circuit breaker will take too long to trip, if it trips at all.

 

Calculating Fault Loop Impedance and Circuit Breaker sizing

Fortunately, the calculations you need to do for a circuit are not difficult. There are tables in AS3008 that give you the resistance of cables of different sizes.

 

Example:

4mm2 copper wire has a resistance of 0.0046 ohms per metre. If circular cable was used the earth wire would be 2.5mm2, which has a resistance of 0.074 ohms per metre. The cable run is 100m.

 

Total resistance = Resistance of Active + Resistance of Earth

                 = (100 x 0.0046) + (100 x 0.0074)

                 = 0.46 + 0.74

                 = 1.202 ohms

 

Remember that to get the total resistance you need to add the resistance of the active cable with the resistance of the earth cable. In this example the earth wire had the same resistance as the active because they were the same size, but in most cables this is not the case.

 

When you have the resistance of the cable run you can calculate how much current will flow through the circuit if there is a short circuit at the end of the cable.

 

Ohms law states that V = I x R

Therefore, I = V / R

 

In this case, V is 240V (your typical Australian phase voltage, but you can substitute if you are using a different voltage) and R is the total resistance of the cable.

 

With this current value you can see whether there will be enough current flowing through your circuit to trip the circuit breaker. For a typical C curve circuit breaker the tripping current is 7.5 times the rated current.

 

Example:

The cable run in the example above is going to be used to run a 7.5kW motor which will draw 14A. It is going to be DOL and so will require a 40A circuit breaker to start.

 

On the surface this circuit appears to be fine. The voltage drop will be 14A x 0.741 ohms = 10.3V, which is less than the maximum 12V drop allowed. The earth resistance is less than 0.5ohms, which is the maximum earth impedance stated in AS3000. The maximum current that 4mm2 can take (touching) is 29A - which is way above the 14A load.

 

However, the short circuit current is a different matter. The current that would flow in a short circuit can be calculated as follows:

    I = V / R

      = 240 / 1.202

      = 199A

 

The circuit breaker will trip in an adequate time when there is more than 7.5 times its rated current flowing through it. In this case:

Trip current = 40A x 7.5

             = 300A

 

 

As can be seen by the above example, there will not be enough current flow to trip the circuit breaker in an adequate timeframe.

 

What to do if Circuit Breaker won't trip

If you are faced with this situation there are only 2 things you can do.

 

The first is to downgrade the rated size of the circuit breaker. Often this is not possible due to the current draw of the load, but sometimes it is. If the load is a motor, consider installing a soft starter.

 

The second solution is to upgrade the cable size. This will reduce the resistance, which means that more current will flow through the circuit when there is a phase to earth short circuit.

 

But AS3008 says I can use a cable that size

This is the most common remark from electricians when they have incorrectly sized cables. They have consulted AS3008 and found that the cable can withstand the current they want to put through the cable.

 

However, this is really only a secondary consideration when designing a circuit. Just because the cable can withstand the current draw during normal conditions does not mean that it can withstand the current draw during fault conditions. The purpose of a circuit breaker is to protect the cable from damage. If the circuit breaker does not trip, an excessive amount of current can flow through the cable, heating it up until it melts or catches fire.

 

But for most circuits there is an RCD

In most domestic situations an RCD would be fitted, and so a short circuit to earth would be detected well before anything major happened.

 

What most electricians do not know is that studies have shown that RCDs have an average failure rate of 7.1% after a prolonged period of time. This is a 1 in 14 chance that the RCD will not work. Circuit breakers do not have anywhere near this failure rate.

 

Regulations requiring the installation of RCDs to circuits were merely intended to be a backup safety measure.

 

Summary

Having too large of a fault loop impedance on a circuit can create very dangerous situations during fault conditions. Design every one of your circuits with this in mind. Ignoring this may unnecessarily jeopardise someone else's life.