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Using RCBO to overcome high Zs but what about short circuits?

Sparkymania
This is something that occurred to me a while ago.

If the Zs is too high for the breaker to operate we ether install a downrated breaker is possible or an RCBO.

My supervisor said something about if an RCBO was installed for this reason then having to test for PSCC at the end of circuit but couldn't say why. He had been told this by NICEIC but no explanation as to why. I argued that this didn't make sense as PFC is only needed at the point where the protective device is and if it's lower at the DB than that of the PD then it will be even lower the further you go down the circuit. I've seen a video that has explained the reason. It's the current flow to trip the breaker on a short circuit rather than an earth fault. Obvious really when you think about it.

However, why would you use a max Zs for line to earth to make sure the resistance allows current flow to trip a breaker for a line to earth fault but not use the same for a line to neutral fault? Surely the impedances would be the same. You already have the max Zs to hand in many tables. With the PSCC you have to calculate what current is needed to trip the breaker or check the tables in the regs. Why do it one way for one and the other way for the other?

Therefore, if you had a C32 MCB with max Zs of 0.54 that would be the same for the line to neutral impedance. Why not just do a Ze then a Z line-neutral.

I see a problem with this as well. RCBOs are often used for higher measured Zs than max Zs if the breaker can't be downrated. But a lot of the time the impedance between line and earth and line and neutral can be almost the same and in a circuit wired with the same size CPC both impedances can be the same. So using and RCD to overcome Zs that's over the max still leaves a problem with a short circuit. This is never taken into account.

This is more an issue with thermal contraints than with shock protection.


I searched the post but couldn't find anything with this subject, although it might be there and I've used the wrong search words.
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  • 0
    There is an important point to remember here and that is that Zs is calculated on non RCD circuits to give a disconnection time of 0.4 seconds, and it takes quite a bit more resistance to take this number up to 5 seconds, which is about when you should start to think there may be a short circuit problem. Remember too that T&E circuits have a reduced CPC size so again you have an additional 30% or so of length to get even to 0.4 seconds disconnection time. It is quite unusual to find a final circuit with a problem, but if the Zs is higher than expected you may well have a CPC problem, so check the L-N loop resistance to check what is wrong. If T&E then remember the 30%, if singles in plastic then CPC should be the same as a live conductor. If in metal containment the CPC (actually the containment) should be lower in resistance than a live conductor. So fairly straight forward really.
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  • 0
    There is an important point to remember here and that is that Zs is calculated on non RCD circuits to give a disconnection time of 0.4 seconds, and it takes quite a bit more resistance to take this number up to 5 seconds, which is about when you should start to think there may be a short circuit problem. Remember too that T&E circuits have a reduced CPC size so again you have an additional 30% or so of length to get even to 0.4 seconds disconnection time. It is quite unusual to find a final circuit with a problem, but if the Zs is higher than expected you may well have a CPC problem, so check the L-N loop resistance to check what is wrong. If T&E then remember the 30%, if singles in plastic then CPC should be the same as a live conductor. If in metal containment the CPC (actually the containment) should be lower in resistance than a live conductor. So fairly straight forward really.
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