If you've had electrical work done, you may be puzzled by the meaning of all the numbers on the certificate your electrician produced.
Under British Standard BS7671, electricians are required to thoroughly test all aspects of electrical systems. This guide gives a basic explanation of what these tests are and why they're done.
Disclaimer: this page is for non-experts (such as householders) who are interested in finding out more about the basics. It's not aimed at professional electricians. We get a huge number of hits which (judging by the search terms) seem to come from professionals. If you're a professional electrician who is not already totally au fait with this material, you should question whether you're competent to do the testing; and if you're just after a quick refresher, read the relevant chapters of the IET On-Site Guide, which covers the topic in far more detail.
For simplicity I've tried to use layman's terms where possible. For example, I refer to the earth wire in a circuit, although this is correctly termed the CPC (Circuit Protective Conductor). Refer to the glossary if in doubt.
External earth fault loop impedence, Ze
This is a measure of the resistance of the wiring between your Consumer Unit (circuit breaker board or fuse board) and the supplier's transformer, which may be some way away. It needs to be high enough to stop a fault current from damaging distribution equipment, but low enough that protective devices (eg your main 100A fuse) will break the circuit if a high-current fault occurs.
Prospective Fault Current, Ipf
This is calculated based on the voltage of your supply and the Ze measurement, and is a theoretical measure of how much current could flow if there was an earth fault (current going directly from live to earth) or short circuit (current going directly from live to neutral).
Satisfactory results depend on the type of electrical supply you have.
Continuity of Ring Circuits (r1, r2, rn)
For ring circuits, the resistance along the length of the ring is measured for each of the live (r1), earth (r2) and neutral cables (rn). This will show any break in the ring, or any junction where the resistance is high, for example where a cable is not properly joined.
Continuity of Protective Conductors (R1+R2, R1+Rn)
For each outlet on a circuit, the resistance of both the live and earth conductors together (R1+R2) is measured back to the source. On a radial circuit, this generally gets higher the further away you get from the source, as the cables have more resistance as they get longer. This test highlights any faults in cabling or connections.
On a ring circuit, the live and earth are cross-connected at each end of the ring, and the R1+R2 measurement will generally be the same at each point on the ring, unless a fault is present. This also therefore helps identify points on a ring where a spur has been connected.
This test is usually repeated for the resistance of both the live and neutral conductors together (R1+Rn), although the result is generally not recorded on testing forms.
The conductors are normally tested in pairs because it's more straightforward to perform the test without trailing test wires all round the property. In some cases, instead of testing R1+R2, the resistance of R2 (earth) will be measured from source to outlet directly.
This test also helps identify polarity problems - for example, if a live wire has accidentally been switched with a neutral wire somewhere in a circuit. It also sometimes identifies socket outlets where a high resistance has built up in the socket itself, for example where a bad connection has blackened or damaged a terminal. It's easy then to replace the socket to solve the problem.
This test has to be carried out with every single appliance and indicator light disconnected from the circuit, which can sometimes involve a lot of detective work!
Initially a 250V direct voltage is applied across the live and neutral wires of the circuit. If a current flows, this indicates something is still connected to the circuit; this has to be tracked down and eliminated. If very little current flows, the test is carried out again at 500V. The goal is for the current to be minimal, indicating that the live and neutral wires are very well insulated from each other.
The test is then carried out between live and earth wires.
A low resistance reading indicates something is bridging between the wires, or insulation is damaged, for example by a floorboard nail driven through part of a cable.
Earth Fault Loop Impedence, Zs
For each circuit, the resistance of the circuit in the event of a short circuit (live-neutral) or earth fault (live-earth) is measured, or if not safe to do so, calculated based on the Ze measurement and R1+R2 measurement. This ensures in the event of a fault, circuitry is not overloaded before a fuse or circuit breaker isolates the fault, and that the fault is isolated within a specified time, based on the circuit type and supply type. Acceptable Zs values depend on the protection type and circuit rating.
If you have RCDs, these are also tested, initially by pressing the button to ensure the RCD trips normally, and then:
Fault Current test, IΔn @ 0° and 180°
The testing device allows a small fault current (normally 30mA for most domestic RCDs) to flow between live and earth wires. This should trigger the RCD within a set time. The test is carried out on both separate phases of the AC supply.
5x Fault Current test, 5IΔn @ 0° and 180°
The testing device then allows a larger fault current (normally 150mA) to flow. This should trip the RCD more quickly, again within a predetermined time for the type of RCD and supply.
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