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RCD fault protection: RA × IΔn ≤ 50 VWhich RCD for an 87 ohm TT barn?
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The second job is fault protection. An RCD is used for fault protection in two situations:
- The installation is part of a TT system.
- The circuit's disconnection time is longer than the maximum allowed.
Both are covered in Section 411 (automatic disconnection of supply).
BS 7671 · Section 411 · automatic disconnection of supply (TT systems and maximum disconnection times)
Most circuits have a maximum disconnection time. Say a circuit on a TN-C-S system would take 7 seconds to disconnect. That's because of a high resistance on the R1 + R2 side, or a high Ze. Either way, if Zs is too high, the fault current is low and disconnection is slow. An RCD for fault protection fixes this.
A common mix-up is that maximum disconnection times range from about 0.1 to 2 seconds. For 230 V, Table 41.1 gives 0.4 s (TN) and 0.2 s (TT) for final circuits up to 63 A with socket-outlets or up to 32 A supplying fixed equipment. Distribution circuits and larger final circuits are allowed 5 s (TN) or 1 s (TT).
Any size will do, as long as it passes
For fault protection, the RCD can be any rating: 30, 100, 300 or 500 mA. Each has a maximum Zs:
| RCD rating IΔn | In amps (use this in the sum) | Maximum Zs |
|---|---|---|
| 30 mA | 0.03 A | 1667 Ω |
| 100 mA | 0.1 A | 500 Ω |
| 300 mA | 0.3 A | 167 Ω |
| 500 mA | 0.5 A | 100 Ω |
BS 7671 · Table 41.5 · maximum Zs for RCDs (1667 / 500 / 167 / 100 Ω)
BS 7671 lets you pick whichever one you like, as long as it passes this check:
RA can be swapped for your measured Zs. IΔn is the RCD's rating in amps: 0.03, 0.1, 0.3 or 0.5. Don't type in 30 or 500, or you'll get the wrong answer.
IΔn is written with a triangle (the Greek letter delta), not an "A" or an "E". It's said "I delta n". The maximum Zs values in the table are just 50 V ÷ IΔn. For example, 50 ÷ 0.03 = 1667 Ω.
BS 7671 · Reg 411.5.3 · RA × IΔn ≤ 50 V for TT systems
Example 1: commercial lighting circuit, Zs = 10 Ω
A lighting circuit in a commercial office on TN-C-S. Measured Zs = 10 Ω.
(10 Ω means something is seriously wrong, but go with it.)
Does it need additional protection?
✗ Not a socket-outlet
✗ Not a domestic lighting circuit
✗ Cables are on the surface in conduit and trunking (you can see them),
so 522.6.202 and 522.6.203 don't apply. They're for hidden cables.
→ No 30 mA requirement. It needs FAULT protection only.
Check each size: Zs × IΔn ≤ 50 V
30 mA : 10 × 0.03 = 0.3 V ✓
100 mA: 10 × 0.1 = 1 V ✓
300 mA: 10 × 0.3 = 3 V ✓
500 mA: 10 × 0.5 = 5 V ✓
→ Any of them is allowed.Example 2: lighting in a barn on TT, Zs = 87 Ω
A lighting circuit in a barn in the middle of a field, on a TT system.
TT → it needs an RCD for fault protection. Measured Zs = 87 Ω (on a Monday).
500 mA: 87 × 0.5 = 43.5 V ✓ but sailing too close to the wind (max 100 Ω)
300 mA: 87 × 0.3 = 26.1 V ✓ still too close for comfort (max 167 Ω)
100 mA: 87 × 0.1 = 8.7 V ✓ our choice (max 500 Ω)
30 mA: 87 × 0.03 = 2.61 V ✓ but more sensitive than it needs to be
Why not the 500 or 300? Ground dries out:
End of a hot week Zs ≈ 107 Ω → 500 mA fails (107 × 0.5 = 53.5 V ✗)
A month with no rain Zs ≈ 207 Ω → 300 mA fails (207 × 0.3 = 62.1 V ✗)
100 mA still fine (207 × 0.1 = 20.7 V ✓)Don't pick a rating whose maximum Zs is close to your reading. Earth electrode resistance rises as the ground dries out. The 100 mA is a good usual choice: it's not too sensitive, and it leaves lots of headroom. Above about 200 Ω an electrode reading may not be stable, so look at improving the electrode too.
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