Electrical Circuit Faults

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There are many types of fault that can occur in electrical circuits. These include not only failure of some physical component but also errors in the original design of the circuit. This article focusses on hard wiring faults that can occur either during normal use, or can be a result of poor workmanship during installation. Although some of the information here be also applicable to lighting circuits, the particular focus of this article are faults on general purpose power circuits (i.e. circuits supplying 13A sockets).

Please note that this document contains information that may be specific to UK wiring regulations and practice. If you are not in the UK, then take great care using this document since there may be fundamental differences to your local wiring practice that could make advice given here inappropriate or even dangerous.

See also Electrical safety

Types of fault

High resistance connections

A high resistance connection can occur in a circuit anywhere a cable or wire is joined. This will usually be at an accessory such as a socket-outlet, switch, light fitting, or junction box.


The most common cause is simply a loose screw terminal connection. This may be because it was not made sufficiently well in the first place, it could be that dirt or other debris is present in the connection (remnants of insulation for example), or it might be that it has become loose over time. Connections that work loose can result from the normal thermal cycling of a circuit. If it routinely carries a significant proportion of its maximum design current, the cable will be subject to repeated heating and cooling cycles. This in turn results in expansion and contraction which can loosen the terminal's grip on its conductor(s). Environmental issues like vibration can also play a big part here. Good cable support and fixing can help mitigate these problems.


The most common effect of a high resistance connection will be localised heating around the connection. On a high current circuit even a small unwanted resistance (of the order of an ohm) can result in the dissipation of hundreds of watts of power at the joint. This will quickly damage the insulation of cable. The follow-on risks are that of fire, or circuit failure, or unexpected operation of the circuit protective device due to short circuit resulting from insulation failure.

A secondary effect can be that of excessive voltage drop experienced in other parts of the circuit. This can result in equipment damage, failure of protective devices to operate correctly, and flickering or variation in brightness of lamps etc.

Broken and disconnected conductors

Less common than high resistance connections are completely open-circuit connections.


These can occur because a wire is no longer (or never was) making contact with the terminal, or has broken or been damaged. The most common wire in a circuit to suffer this problem is the earth (or Circuit Protective Conductor (CPC) as it is formally known). This is because this wire is typically smaller than the others (in T&E cable), and it is usually unsheathed and hence requires protection by a slip-on piece of sleeving. Smaller wires are easier to break by over-tightening of terminals, and are more likely to not ever be connected in the first place as the presence of the loose sleeving can obstruct visibility of the actual conductor which can make it more difficult to position and hold the wire in place in the terminal when tightening.


On live and neutral connections a broken conductor will either stop a circuit working correctly or may result in a risk of overload in a part of it.

In the case of a broken CPC the risks can be much greater, since it could impair or prevent operation of the protective devices in the circuit, and could expose someone to a serious shock hazard.

Worn and defective accessories

Accessories have a limited life span. This in particular applies to sockets. Over time the terminals will get dirty, and can lose some of their spring tension. This will cause high resistance connections. The knock-on effect of local heating can further damage the socket. Switches may also get dirty and develop resistance, or simply break and fail to switch any more leaving the accessory permanently stuck in one position or the other.

Fault and overload currents

The terms "Fault current" and "Overload Current" have specific meanings in the context of the wiring regulations.

Fault Current

This is the current that flows when a short circuit fault occurs in a circuit between either the live and neutral conductors, or the live and earth conductors. The magnitude of fault currents can be huge (100s or 1000s of Amps) since they are limited only by the resistance of the wires in the circuit between the consumer unit and the fault, and the impedance of your power supply and earth connection as delivered to the property.

Overload Current

This occurs when the total current demand made by the appliances connected to the circuit exceed its design capacity. This significance of an overload will depend on its magnitude and its duration. Small overloads may be tolerable for long durations, while big ones can only be tolerated for short durations, before any damage to the circuit wiring occurs.

Circuit protection

It it typically the responsibility of the circuit protective device to deal with both fault and overload currents safely. However the responsibility can be split between more than one device if required. The fault protection must always be provided at the origin of the circuit, but the overload protection may be dealt with separately in some circumstances.

Modern Miniature Circuit Breakers (MCB) include two separate mechanisms, one to deal very high current demands that are typical of a fault condition, and another to deal with more drawn out but lower current demands that are in excess of the circuit's design capacity. The "instantaneous" mechanism will cause a magnetic solenoid to open the device immediately (within 0.1 secs). A slower, thermal, mechanism will open the device after a period of overload which may range from seconds to hours.

Fuses behave in a similar way to to MCBs for sustained overload but are usually not as fast to respond to fault currents as MCBs.

Effects of circuit faults on different circuit types

Circuits can be divided into two broad categories: Ring final circuits (often incorrectly referred to as "Ring Mains") and Radial circuits. The effects of the faults described above can be different in each circuit type, and also the safety implications are different.

A common question is: which type of circuit is "safer" in the event of a fault. The answer is not straightforward and depends on circumstance. Bold statements that one type is better than the other "because" are usually rather simplistic and overlook important details.

An important consideration is also the presence of a Residual Current Device (RCD) circuit breaker. If one of these is also protecting a circuit, then the dangers associated with high resistance or disconnected protective conductors are greatly reduced, and hence why their use on general purpose power circuits is to be encouraged.

Radial circuits

High resistance connection faults

Generally radial circuits do not perform well with this type of fault.

The severity of the problem caused by a high resistance connection in a radial will depend on where it is. The worst case is a fault near the supply end of the circuit where the conductors are liable to be carrying the greatest current, supplying all outlets on the radial. A fault in the live or neutral connection will result in heating. A fault in the CPC will result in a reduction in safety of the circuit since operation times of protective devices in the event of a fault can extend, or they may fail to operate altogether, and the potential for dangerous voltages to be present on metal casework of appliances increases (i.e. a shock hazard caused by "indirect contact").

Broken and disconnected conductor faults

A break in the live or neutral will stop outlets downstream from working, giving a good indication that a fault is present (and making the fault relatively easy to locate and correct).

The more common event of a break in the CPC however will cause a particularly dangerous failure mode. The circuit will continue to supply power and appliances will appear to function as normal, but much of the circuit's fault protection has been lost and the shock risk from indirect contact during a fault is severe. There are not usually any obvious indications of this fault condition to a normal user.

Ring final circuits

High resistance connection faults

Since there is an alternative conduction path (the "other way" round the ring) the risks from a high resistance connections in the live or neutral wire are dramatically reduced. Typically (but not always) this failure can result in an increased chance of a low level overload occurring elsewhere in the ring since the cable used is typically rated to carry a bit less than the full circuit load.

The ring circuit of course makes no difference to a bad connection outside of the ring, such as damage inside a socket. Such a fault poses the same risk regardless of the wiring feeding it.

The same high resistance fault in the CPC is handled very well by a ring circuit of a modern design. There is generally no immediate reduction in safety and the impairment of the operation of protective devices should be minimal. Ring circuits of old design that are not protected by MCB or RCD and often with a thinner CPC in the circuit cable are a little different: the risk is considerably reduced by the ring configuration, but some risk of failure to clear a fault remains, due to the possibility of the remaining CPC melting.

Voltage drop related problems are also likely to be less severe (but hence also less noticeable).

Broken and disconnected conductor faults

In the case of a break in the live or neutral conductor a ring circuit will continue working but with a risk of (usually) low level overload in some parts of the circuit (although fault protection is usually adequately maintained in this circumstance].

A broken CPC has little effect on a ring circuit of a modern design, and it will usually continue to operate normally in most cases without having much effect on the performance of the protective devices, or the risk of shock from indirect contact. Again, on older circuit designs, using rewireable fuses, and a thinner CPC the risks are greater.

Detecting circuit faults

There are three basic ways of detecting circuit faults:

  1. Change in behaviour or performance of the circuit (including stopping working!)
  2. Inspection of the wiring
  3. Testing

The irony is that (3) is probably the most effective way to find any problems, but (1) is the one most likely to happen in practice.

User observable changes

The following table outlines circuit behaviours that may realistically be observed by a circuit's end user without any detailed inspection of the wiring, and without carrying out any tests.

It is important to note that there are a number of circuit failures for which there will be no directly observable behaviour, unless there is also another fault present on the circuit (at which point the circuit may become immediately dangerous).

Observable behaviour indicating the fault
Radial Circuit Ring Circuit
High resistance L/N
  • Heating localised to socket position
  • Burning / Fishy / Urine smell from hot thermosetting plastics
  • symptoms of low voltage at downstream sockets (dim lamps etc)
  • Fire
  • No observable change
High resistance CPC
  • No observable behaviour in the absence of another fault
  • Shock risk if the right/wrong appliance fault occurs & no RCD fitted
  • No observable behaviour in the absence of another fault
Disconnected L/N
  • Failure of all or part of circuit to provide power
  • No observable behaviour
Disconnected CPC
  • No observable behaviour in absence of another fault
  • Very high shock risk in the event of a fault
  • No observable behaviour
  • With a ring circuit to a modern design (i.e. MCBs used for circuit protection, and cable with a 1.5mm² CPC) a good degree of circuit safety will usually be maintained in all but the most extreme circumstances (The worst case would be when the break in the CPC is at the very start of one leg of the ring, and the fault is at the start of the other leg).
  • With older installs that are protected by BS 3036 rewireable fuses, and use cable which only has a 1mm² CPC, the risks are greater, with more serious cases likely to result in a failure to clear a fault in an acceptable time, and the risk of live voltage being maintained on earthed metalwork connected to the circuit for several seconds if a live to earth fault occurs.

Inspection procedure

The following table outlines inspections that can be carried out to identify various fault classes. These apply to both Radial and Ring circuit. Note all these inspections tests will require the dismounting (at least some of) the accessories (switches / sockets etc) on the circuit to enable the inspection to take place.

Power must be switched off before any inspection takes place.

Inspection procedure
Look for do
High resistance L/N
  • Charred, discoloured, or burnt insulation
  • Discolouration or melting of plastics
  • Blue heat discolouration of terminals
  • Check tightness of screw terminals with a screwdriver
  • Pull individual wires to check they don't pull free from terminals and crimp connections
High resistance CPC
  • Nothing to see
  • Check tightness of screw terminals with a screwdriver
  • Pull individual wires to check they don't pull free from terminals and crimp connections
Disconnected L/N
  • Broken, damaged, or unterminated wire
  • Pull individual wires to check they don't pull free from terminals and crimp connections
Disconnected CPC
  • Broken, damaged, or unterminated wire
  • Check green/yellow sleeving is not obscuring a unconnected wire end
  • Pull individual wires to check they don't pull free from terminals and crimp connections

Test procedures

This section contains a limited subset of tests that can be used to identify the particular circuit faults listed here. Note that these tests are not exhaustive, and are not adequate to commission a new circuit for the first time. For a full list of the tests that should be carried out please see the IEE Wiring Regulations On Site Guide Section 10.

All these test must be run with the power to the circuit turned off

Required Equipment

Test Meter

For some basic checks on circuits and a simple yes / no style continuity check then a basic multimeter will be adequate. Either a simple analogue one like this simple multimeter or its digital equivalent is required.

To carry out the tests that require accurate measurements of low resistance connections then the ideal tool is a specialised circuit tester such as [this]. However these are quite expensive. A reasonable quality digital multimeter that can measure resistances with an accuracy of 0.1 ohms or better will also be able to give useful results for many of the tests. There are a huge range of meters that fit this criteria. Some are shown [here]

Socket Tester

A basic socket tester is also invaluable. For some tests a 13A plug is also useful.

(Many of the checks done here with the socket tester could be duplicated with the multimeter but with more complexity).

To minimise risk to the tester, these tests are described in a way that does not require the consumer unit to be opened.

Shorting Plug

To aid testing a shorting plug can be used. This is an ordinary 13A plug, fitted with a 3A fuse, that has an internal link between the live and neutral connections and the live and the earth connections, and no external connections at all.

Note this must never be plugged into an energised circuit, and should be clearly labelled with a warning to this effect. Also note that it will probably cause any RCD that is protecting the circuit under test to trip - even if the fuse is removed, or MCB switched off. One should always ensure power is turned off at the main switch. Failure to turn the power off at the main switch will also invalidate some of the resistance tests described here

Taking Measurements

When making resistance measurements with the multimeter, use the meter's lowest resistance (Ohms) range. First measure the resistance reading with the probes shorted together. If the meter has a zero adjust facility then use this to eliminate the probe resistance. If it has no adjustment capability then subtract this reading (the probe resistance) from measurements made on circuits under test.

Detecting basic circuit faults
Radial Circuit Ring Circuit
High resistance L/N
  • Locate the first and last sockets on the radial
  • At the first socket insert the shorting plug
  • At the last socket, use the multimeter to measure the resistance between live and neutral on a low resistance range. On a non-faulty circuit the resistance will be typically be less than one ohm. The actual value measured should be approximately equal to the length of cable between sockets multiplied by the 'L/N round trip' resistance per metre value given in the table below.

Example: For 10 metres of 2.5mm² T&E cable you can expect the round trip resistance from Neutral to Live will be:

10 x 0.01482 = 0.1482 ohms

  • If the resistance is measured is noticeably higher than expected then this is a good indication of a high resistance fault. The location of the fault can be found by moving the shorting plug to the next socket in sequence on the radial and measuring the resistance again. When the resistance drops to an expected level this indicates that the fault is probably at the connections in the socket that the shoring plug is currently plugged into, or the previous one. Use the inspection methods described in the table above to identify the fault.
  • Disconnect a socket on the ring (not on a spur from it)
  • Measure the resistance between the two live wires, and between the two neutral wires. Check to see if they are equal, and also match values expected based on the estimated length of the circuit cable and the resistance table. A high reading on one or both sets of wires probably indicates a fault.
  • The fault location can be narrowed down by measuring resistance between Live and Neutral with the shorting plug connected to another socket on the ring. Move the shorting plug and repeat measurements to narrow down the location. Fix as described for the radial circuit.
High resistance CPC
  • Test as described for a high resistance L/N fault, except by measuring between Live and Earth wires. Note that most T&E cables do not have equal size earth and main conductor wire sizes. This has to be taken into account when calculating expected resistances, so use the wire resistance values from the "L/N + CPC Round trip" column of the table.


20m of 2.5mm² cable between test point and shorting plug would give 20 x 0.01951 = 0.39 ohms.

  • Note that if this test is not carried out with both live and neutral disconnected at the consumer unit (i.e. with the main switch off) the results will be invalid.
  • As per radial test.
Disconnected L/N
  • Use the plug in socket tester in each socket on the circuit to locate this fault.
  • As per radial test.
Disconnected CPC
  • Use the plug in socket tester in each socket on the circuit to locate this fault.
  • As per radial test.

Wire resistance table

Wire CSA/CPC (mm²) mOhms / metre L/N round trip (mOhms) L/N + CPC Round trip (mOhms)
1.0 / 1 18.10 36.20 36.20
1.5 / 1 12.10 24.20 30.20
2.5 / 1.5 7.41 14.82 19.51
4.0 / 1.5 4.61 9.22 16.71
6.0 / 2.5 3.08 6.16 10.49
10.0 / 4 1.83 3.66 6.44
16.0 / 6 1.15 2.30 4.23

Table Notes

  1. The Wire Cross Section Area (CSA) column also indicates the typical CSA of the CPC wire used in a modern cable.
  2. The mOhms / Metre value indicates the resistance of a single length of the main L or N conductor in a cable
  3. The L/N round trip value is simply double the single length value. It indicates the expected resistance reading you would get measuring between L and N when you have placed a short at the far end
  4. The L/N + CPC value is similar to (3) but instead indicates the round trip resistance expected if measuring between L or N and CPC with a short between these at a distant point (the CPC typically being thinner than the main conductors raises the value).

Repairing circuit faults

Faults arising from disconnected or broken wires can usually be fixed simply by re-terminating the wire correctly at the accessory position. If however the fault was a high resistance connection care must be taken to ensure that damage has not occurred due to the localised heating effect. Any accessory showing signs of heating damage should be replaced. Any damaged wire should be cut back to sound wire before re-terminating. If this makes the wire too short then an extension piece should be Crimped into position first.

If it is not possible to assess if the cable insulation is damaged do to it being obscured in walls etc, then a full insulation resistance test should be carried out using a high voltage insulation resistance tester. Any cable segments that indicate failing resistance should be replaced.

Testing repairs

Once any repairs have been made to the circuit it is important to retest it to verify that the fault is gone and that you have not introduced any others in the process.