Topic 4 of 5
Earthing, bonding and disconnection times
Earthing and bonding work together so that a fault causes a large enough current to trip the protective device quickly, and so that touchable metalwork doesn't reach a dangerous voltage.
Automatic disconnection of supply (ADS)
ADS is the main way BS 7671 protects against electric shock. If a line conductor touches earthed metalwork, fault current flows round the earth fault loop: line conductor, the fault, the circuit protective conductor (CPC), the main earthing terminal, the supply earth, and back through the transformer. The protective device sees the large current and disconnects.
For this to happen fast enough, the loop's total impedance (Zs) must be no more than the maximum for that device. Zs = Ze + (R1 + R2).
Disconnection times (TN systems, 230 V)
| Circuit | Maximum time |
|---|---|
| Final circuits up to 63 A with socket-outlets, and up to 32 A feeding fixed equipment only | 0.4 s |
| The same final circuits on a TT system | 0.2 s |
| Distribution circuits, and larger final circuits, on TN systems | 5 s |
The parts
- Main earthing terminal (MET): where all the protective conductors meet, usually in or next to the consumer unit.
- Earthing conductor: connects the MET to the means of earthing (the supply earth or an earth electrode).
- Circuit protective conductor (CPC): the earth in each circuit, connecting exposed-conductive-parts back to the MET.
- Main protective bonding conductors: connect extraneous-conductive-parts, such as metal water and gas pipes where they enter the building, to the MET.
Exposed-conductive-parts are metal parts of electrical equipment that could become live in a fault, such as a metal light fitting. Extraneous-conductive-parts aren't part of the electrical installation but could bring in a voltage, usually earth potential, such as metal pipes.
Main protective bonding sizes
- TN-S and TT: at least half the size of the earthing conductor, with a minimum of 6 mm². It needn't be more than 25 mm² (copper).
- TN-C-S (PME): sized from the supply neutral (BS 7671 Table 54.8). For a neutral up to 35 mm², which covers most homes, the minimum is 10 mm² copper.
Exam tip: bonding doesn't make anything disconnect. It keeps metalwork at the same voltage so there's no dangerous difference to touch across while the device trips.
Try it: Maximum Zs calculator
Mini test
6 questions. Pick an answer to see if you're right and why.