Question 1

What disconnection time applies to a 63A cooker circuit with a socket outlet?

Accepted Answer

Under the 18th Edition, any final circuit rated up to and including 63A that includes socket outlets requires 0.4s disconnection in a TN system. This was a key change from the 17th Edition, where circuits above 32A were often treated as distribution circuits with a 5s allowance.

Question 2

What's the difference between Type B, C, and D MCBs for disconnection?

Accepted Answer

The letter indicates the magnetic trip range. Type B trips at 3-5 times In, Type C at 5-10 times In, and Type D at 10-20 times In. A lower trip multiple means the MCB needs less fault current to disconnect quickly, which translates to a higher permissible Zs. Type B is therefore preferred where fault levels are lower.

Question 3

My Zs just passes but is close to the maximum — should I be concerned?

Accepted Answer

Yes. The tabulated maximum Zs values in BS 7671 are already reduced to 80% of the theoretical maximum to account for conductor temperature rise during a fault. However, Zs increases with temperature, so a reading taken on a cold circuit may exceed the limit when the circuit is fully loaded. Allow a safety margin.

Question 4

Why does a TT system require faster disconnection times?

Accepted Answer

In a TT system the earth return path has much higher impedance than in a TN system, because it relies on the general mass of earth rather than a metallic return conductor. This higher impedance means lower fault currents, so the protective device may not operate quickly enough without an RCD. The tighter time limits reflect this additional risk.

Question 5

Can an RCD compensate for high Zs values?

Accepted Answer

Yes, to a degree. A 30mA RCD will trip at just 30mA of earth fault current, so even with high Zs the RCD provides rapid disconnection. However, BS 7671 still requires the Zs to be low enough for the overcurrent device to clear a line-to-neutral fault. RCDs only detect earth leakage, not all fault types.

System	Circuit Type	Max Time (s)
TN	Final ≤63A (socket outlets)	0.4
TN	Final ≤63A (fixed equipment)	0.4
TN	Distribution circuits	5
TT	Final circuits	0.2
TT	Distribution circuits	1

MCB Type	Trip Range (× In)	Typical Use
Type B	3–5	Domestic, resistive loads
Type C	5–10	Small motors, fluorescent lighting
Type D	10–20	Large motors, transformers, X-ray equipment

Disconnection Time Calculator

How This Calculator Works

Maximum Disconnection Times

MCB Magnetic Trip Multiples

Practical Notes

Frequently Asked Questions

What disconnection time applies to a 63A cooker circuit with a socket outlet?

What's the difference between Type B, C, and D MCBs for disconnection?

My Zs just passes but is close to the maximum — should I be concerned?

Why does a TT system require faster disconnection times?

Can an RCD compensate for high Zs values?

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