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One installation, five circuits: disconnection times worked throughTable 41.1 or not, circuit by circuit?
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Our example installation
To cover every case, we'll use one installation with five circuits. It's on a TN-S earthing arrangement, which is common in the UK. It doesn't matter whether it's domestic, commercial or industrial. The rules are the same.
- The supply enters the building and feeds DB1.
- DB1 has two final circuits (red and green) and one distribution circuit (yellow) that feeds DB2.
- DB2 has two final circuits (blue and purple).
Final circuit or distribution circuit?
- Final circuit
- A circuit that is finally being used: it supplies the lights, the heaters, the cooker, the shower or the socket-outlets directly.
- Distribution circuit
- A circuit that feeds another distribution board (like the yellow submain from DB1 to DB2). It doesn't supply the equipment itself.
BS 7671 · Part 2 Definitions · "Circuit, final" and "Circuit, distribution" Worth a read. Check your copy for the page
Working through the five circuits
For each circuit, ask: Is it a final circuit? Does it supply sockets or fixed equipment? What's its rating?
| Circuit | What it is | Which rule | Max time (TN, 230 V) |
|---|---|---|---|
| 1 · red | Final circuit, sockets, 32 A (a typical ring or radial) | Table 41.1 (sockets ≤ 63 A) | 0.4 s |
| 2 · green | Final circuit, fixed equipment, 40 A (e.g. a shower) | Reg 411.3.2.3 (fixed > 32 A) | 5 s |
| 3 · yellow | Distribution circuit, DB1 to DB2 | Reg 411.3.2.3 (never Table 41.1) | 5 s |
| 4 · blue | Final circuit, fixed equipment, 32 A (e.g. a cooker) | Table 41.1 (fixed ≤ 32 A) | 0.4 s |
| 5 · purple | Final circuit, sockets, 80 A | Reg 411.3.2.3 (sockets > 63 A) | 5 s |
If the installation were TT instead, circuits 2, 3 and 5 would use Reg 411.3.2.4 (up to 1 s), and circuits 1 and 4 would use the TT row of Table 41.1 (0.2 s).
"Fixed equipment" means
Anything that's fixed current-using equipment rather than something that plugs in. In practice it's any circuit that isn't a socket circuit: a shower, a cooker, a heater.
The DB2 trap
Some people think that because DB2 is fed by a distribution circuit, the circuits from DB2 can't be final circuits. Wrong. Circuits 4 and 5 are still final circuits. Look at each circuit on its own, not at what feeds its board.
Sockets above 63 A?
You'll almost never see this in a house, but it happens in commercial and industrial work. A "socket-outlet" isn't only a 13 A 3-pin socket on a wall. It's anything classed as a socket, such as an industrial socket for a large machine that plugs in and draws a high current.
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