Zs values when there is additional RCD protection

arashabb · Oct 22, 2019

Hi everyone,
Consider we have an RCD+MCB protection for a circuit.
if we follow the tables for Zs values of a 10A, MCB, type C, the maximum value shall be 1.75 Ohm, this happens when 100A current flows ( as per MCB breaking curve). When the RCD disconnects the circuit in much less current, why do we still need to measure and verify Zs values?! RCD will never allow the current to reach some where near 100A! so even if the Zs value is much higher than permitted, it does not matter because the RCD will remove the fault. Am I wrong?!

telectrix · Oct 22, 2019

both right and wrong. RCD will disconnect at approx 30mA, but it means that you are relying on a RCD for fault protection, whereas they are meant to be additional protection. fault protection should be provided by the MCB.

arashabb · Oct 22, 2019

telectrix said:
both right and wrong. RCD will disconnect at approx 30mA, but it means that you are relying on a RCD for fault protection, whereas they are meant to be additional protection. fault protection should be provided by the MCB.

Thank you. Dont see the problem on relying on the RCD though but you are right, as per regulation, it shall be used as additional protection.

APE37 · Oct 22, 2019

What would happen if the RCD became faulty?

Ian1981 · Oct 22, 2019

That’s why we test rcd’s.
It’s a non argument to say such a thing as what if the rcd becomes faulty.
If you worried about that then what’s the point of installing one?
How do we know that an mcb is not faulty?
Tho primarily used for additional protection a 30mA rcd will give fault protection also for say TT domestic installations and any other type of installation

arashabb · Oct 23, 2019

APE37 said:
What would happen if the RCD became faulty?

I guess we can ask this question for all protective devices. plus, testing RCD is one of the testing procedures that we do as per BS7671 during routine checks and periodic inspections. I guess by having the RCD, even if the Zs values are slightly higher, everything is still protected by good margin.