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

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 Sparkymania over 5 years ago

Hi gkenyon

We haven't actually started doing these tests. I had never heard of taking PSCC at anywhere apart from the DB before until my supervisor mentioned it, and even then he couldn't explain why, only that the NIC bloke said you should do it if the Zs was to high and you used an RCBO to comply to disconnection times on earth fault. Even the other sparks were confused. It makes sense now that you are checking if the current flowing on that type of fault is enough to operate the PD because if the Ze is too high than the other loops may be too high. But if that's the case using an RCBO will only overcome one problem not the other.

In all the time I've been doing this I, and any other spark I've worked with, has never gone that deep into that part of the regs with those calculations.

The most we do is check Zs measured to Zs max and take whatever appropriate action on that. Until recently, never thought about short circuit measurement issues although they are in the reg you pointed out.

I don't know if this is more a design stage issue.

However, I would have still thought you could use the same principle of line to neutral loop impedance and apply it in the same way as Zs. After all, it would be the same current required to trip the breaker (MCB part) whether the fault was to earth neutral of another line and therefore if within the max Zs for line-neutral it would be ok.

From what I read it seems that the fault can be more that the time you mention. It says "for a fault of less than 1 second) then a certain criteria must be met, not that the fault must be less than one second.

This is now getting more complicated than I thought it would be.
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0 Sparkymania over 5 years ago

Hi gkenyon

We haven't actually started doing these tests. I had never heard of taking PSCC at anywhere apart from the DB before until my supervisor mentioned it, and even then he couldn't explain why, only that the NIC bloke said you should do it if the Zs was to high and you used an RCBO to comply to disconnection times on earth fault. Even the other sparks were confused. It makes sense now that you are checking if the current flowing on that type of fault is enough to operate the PD because if the Ze is too high than the other loops may be too high. But if that's the case using an RCBO will only overcome one problem not the other.

In all the time I've been doing this I, and any other spark I've worked with, has never gone that deep into that part of the regs with those calculations.

The most we do is check Zs measured to Zs max and take whatever appropriate action on that. Until recently, never thought about short circuit measurement issues although they are in the reg you pointed out.

I don't know if this is more a design stage issue.

However, I would have still thought you could use the same principle of line to neutral loop impedance and apply it in the same way as Zs. After all, it would be the same current required to trip the breaker (MCB part) whether the fault was to earth neutral of another line and therefore if within the max Zs for line-neutral it would be ok.

From what I read it seems that the fault can be more that the time you mention. It says "for a fault of less than 1 second) then a certain criteria must be met, not that the fault must be less than one second.

This is now getting more complicated than I thought it would be.
Cancel
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Using RCBO to overcome high Zs but what about short circuits?

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