Electrical Circuit Faults
Circuit Faults
(This is a work in progress that ultimately could include information not only on common faults, but also tracing and fixing them)
Please do not rely on any information in this article yet. It has not been checked, peer reviewed, or even proof read
--John Rumm 05:46, 13 May 2007 (BST)
-- might need a bunch of glossary entries for some of the terms used here
Types of fault
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
High resistance connections
A high resistance connection can occur in a circuit anywhere a cable or wire is joined. This will most usually be at an accessory such as a socket outlet, switch, or light fitting.
Causes
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 work loose a wire. Environmental issues like vibration can also play a big part here.
Good cable support and fixing can help mitigate these problems.
Effects
The most common effect of a high resistance connection will be localised heating around the connection location. On a high current circuit even a small unwanted resistance (i.e. an ohm for example) 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 variation in brightness of lamps etc.
Broken and disconnected conductors
Less common than high resistance connections, are open circuit connections.
Causes
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, are more likely to crack when mechanically moved, and are more likely to not ever be connected in the first place as the presence of the loose sleeving can obstruct the wires visibility of the actual conductor, and also make it more difficult to position and hold the wire in place in the terminal when tightening.
Effects
On live and neutral connections a broken conductor will wither 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.
Inappropriate accessory provision
Number, type, place, suitability for environment (expand - might want to go in a design faults section)
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 the property.
Overload Current
This occurs when the total current demands 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, and big ones for short durations.
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 Micro 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 deamnds that are in excess of the circuits design capacity. The "instantaneous" mechanism will cause a magnetic solenoid to open the device immediately (within 0.1 secs). The slower thermal mechanism will open the device after a period of overload which may range from seconds to hours.
Effects of circuit faults on different circuit types
Circuits can be divided into two broad categories: Ring final circuits (often incorrectly referred to a "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.
Radial circuits
High resistance connection faults
Generally radial circuits do not perform that 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. Near the supply end of the circuit is the place it carries most risk as this is the point on the circuit that carries the greatest current. In the phase ("Live") or Neutral connection this fault will result in heating. In the CPC this 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
Radial performance with this type of fault is a mixed bag!
Radials perform well in the case of a live of neutral break - since a section of the circuit downstream of the break will simply stop working. This is an very good indication that a fault is present and also easy to detect and correct.
In the more common event of a break in the CPC however, radials exhibit a particularly dangerous failure mode. The circuit will continue to supply power, and appliances will appear to function as normal, however much of the circuits fault protection has been lost, and the shock risk from due to 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
High resistance connections in the live or neutral wire will have similar effects as described above for radials. However since there is an alternative conduction path (the "other way" round the ring) this is mitigated somewhat and the immediate risks are usually reduced. Depending on the location of the failure, and the nature of the normal circuit loading (plus installation details of the circuit) this failure can result in an increased risk of an overload occurring elsewhere in a subset of the ring since the cable used is not typically rated to carry the full circuit load.
The same fault in the CPC is handled very well by a ring circuit. There is generally no immediate reduction in safety and the impairment of the operation of protective devices should be minimal.
Voltage drop related problems are also likely to be less severe (but hence also less noticeable)
Broken and disconnected conductor faults
In live and neutral conductions the ring circuit performs less well than the radial in this situation since it appears to carry on working, but there is now an increased risk of overload in some parts of the circuit (note however that fault protection is usually adequately maintained in this circumstance].
A broken CPC has little effect on a ring circuit and it will 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.
Detecting circuit faults
There are three basic ways of detecting circuit faults:
- Change in behaviour or performance of the circuit (including stopping working!)
- Inspection of the wiring
- 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 circuits 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).
| Radial Circuit | Ring Circuit | |
|---|---|---|
| High resistance L/N |
|
|
| High resistance CPC |
|
|
| Disconnected L/N |
|
|
| Disconnected CPC |
|
|
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.
| Look for | do | |
|---|---|---|
| High resistance L/N |
|
|
| High resistance CPC |
|
|
| Disconnected L/N |
|
|
| Disconnected CPC |
|
|
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.
(to be continued)
Repairing circuit faults
Notes
Wire and accessory replacement cable joints (new article by the sounds of it)