Max Zs table for MCBs and RCBOs
Maximum earth fault loop impedance for Type B, C and D circuit-breakers and RCBOs, 6 to 63 A, from BS 7671 Table 41.3. A 32 A Type B (B32) is 1.37 Ω, or 1.10 Ω for a reading taken at room temperature.
Jump to: Type B · Type C · Type D · check a reading with the calculator
Type B circuit-breakers
Homes: sockets, lighting, heaters. Trips instantly at 5 × In.
| Rating | Max Zs (Ω) | 80 % for test readings (Ω) |
|---|---|---|
| B6 (6 A) | 7.28 | 5.82 |
| B10 (10 A) | 4.37 | 3.50 |
| B16 (16 A) | 2.73 | 2.18 |
| B20 (20 A) | 2.19 | 1.75 |
| B25 (25 A) | 1.75 | 1.40 |
| B32 (32 A) | 1.37 | 1.10 |
| B40 (40 A) | 1.09 | 0.87 |
| B50 (50 A) | 0.87 | 0.70 |
| B63 (63 A) | 0.69 | 0.55 |
Type C circuit-breakers
Commercial: small motors, lots of lighting. Trips instantly at 10 × In.
| Rating | Max Zs (Ω) | 80 % for test readings (Ω) |
|---|---|---|
| C6 (6 A) | 3.64 | 2.91 |
| C10 (10 A) | 2.19 | 1.75 |
| C16 (16 A) | 1.37 | 1.10 |
| C20 (20 A) | 1.09 | 0.87 |
| C25 (25 A) | 0.87 | 0.70 |
| C32 (32 A) | 0.68 | 0.54 |
| C40 (40 A) | 0.55 | 0.44 |
| C50 (50 A) | 0.44 | 0.35 |
| C63 (63 A) | 0.35 | 0.28 |
Type D circuit-breakers
Industrial: large motors, transformers. Trips instantly at 20 × In.
| Rating | Max Zs (Ω) | 80 % for test readings (Ω) |
|---|---|---|
| D6 (6 A) | 1.82 | 1.46 |
| D10 (10 A) | 1.09 | 0.87 |
| D16 (16 A) | 0.68 | 0.54 |
| D20 (20 A) | 0.55 | 0.44 |
| D25 (25 A) | 0.44 | 0.35 |
| D32 (32 A) | 0.34 | 0.27 |
| D40 (40 A) | 0.27 | 0.22 |
| D50 (50 A) | 0.22 | 0.18 |
| D63 (63 A) | 0.17 | 0.14 |
How these are worked out
Zs = (Uo × Cmin) ÷ Ia = (230 × 0.95) ÷ Ia
Ia is the current that trips the breaker instantly: 5, 10 or 20 times its rating for Type B, C or D. These values apply to circuit-breakers to BS EN 60898 and RCBOs to BS EN 61009 on 230 V circuits needing 0.4 s disconnection.
Why use 80 %?
The table values are for cables at full operating temperature. When you test, the cable is cold and its resistance is lower, so a common rule of thumb is that your reading should be no more than 80 % of the table value.
TN-S, TN-C-S and TT
The limits above are the same whatever the earthing system. On a TT system the earth fault loop impedance is usually far too high for a breaker to trip in time, so the circuit relies on an RCD instead. Check BS 7671 for the RCD limits.
Check against your copy of BS 7671 or the On-Site Guide before relying on a value. Learn more in Earthing, bonding and disconnection times.