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Disconnection times: choosing the maximum Zs tableA rewireable fuse. Which max Zs table?

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The disconnection time decides which maximum earth fault loop impedance (Zs) table you use when designing. There are three tables, one after another:

TableDevicesDisconnection timePage (A4)
41.2Fuses: BS 88-2, BS 88-3, BS 3036, BS 13620.4 s74
41.3Circuit-breakers and RCBOs (their own table)0.4 s and 5 s75
41.4Fuses: BS 88-2, BS 88-3, BS 3036, BS 13625 s76

BS 7671 · Table 41.2 max Zs, fuses, 0.4 s · p.74 BS 7671 · Table 41.3 max Zs, circuit-breakers and RCBOs · p.75 BS 7671 · Table 41.4 max Zs, fuses, 5 s · p.76

BS 88-2 / BS 88-3
Cartridge style fuses (heavy-duty cartridge fuses)
BS 3036
Rewireable fuses (fuse wire). Still common in a lot of places.
BS 1362
Plug-top fuses (the fuse in a 13 A plug)
Circuit-breaker or RCBO any disconnection time Table 41.3 covers 0.4 s and 5 s Fuse BS 88-2 · BS 88-3 BS 3036 · BS 1362 0.4 s (Table 41.1 circuit) 5 s (Reg 411.3.2.3 circuit) Table 41.2 fuses, 0.4 s Table 41.4 · p.76 fuses, 5 s
Breakers and RCBOs have one table. For fuses, you first need the disconnection time, and that tells you which table to use.

With a circuit-breaker or RCBO, it's simple: go straight to Table 41.3. You should still know the disconnection time and use it carefully in your design. It's with fuses that the disconnection time really matters, because it decides between Table 41.2 and 41.4. The top of each fuse table states the disconnection time it's for: 0.4 s on 41.2 and 5 s on 41.4.

Watch out
A common way to put it is that circuit-breakers and RCBOs "have their own 0.4 disconnection time parameter". More exactly, the heading of Table 41.3 says its values give compliance with both the 0.4 s time (Table 41.1) and the 5 s time (Reg 411.3.2.3). That's why you don't need to pick between two tables for a breaker. Type D is the exception: Table 41.3 gives Type D separate 0.4 s and 5 s columns, so for a Type D the disconnection time still matters.

Example A · rewireable fuse, heating circuit

Circuit: BS 3036 rewireable fuse · final circuit · 20 A heating (fixed equipment)
Final circuit, fixed equipment NOT above 32 A → Table 41.1
TN, AC, U₀ = 230 V → max disconnection time 0.4 s
Fuse + 0.4 s → take max Zs from Table 41.2

Example B · rewireable fuse, feed to a shed

Circuit: BS 3036 rewireable fuse · feeds a consumer unit in a shed
Feeds another board → distribution circuit → NOT Table 41.1
TN → Reg 411.3.2.3 → max disconnection time 5 s
Fuse + 5 s → take max Zs from Table 41.4

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