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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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    Sparkymania:

    Thanks Farmboy


    I have listened to that one. 

    Talks about taking PSCC at the end of circuit but doesn't go into why you can't just use the line-neutral loop impedance and compare it to the max Zs as I would have thought that would be the same. Same fault current to trip MCB whether to earth or neutral.




    You could do that, but what do you do if the loop impedance comes out too high? Does that mean it's a "fail"?

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    If the L-N loop imp. is too high won't that mean the current flow will then be too low, the same as it would be for L-E loop imp.

    We don't test for PEFC to make sure an MCB will trip on an earth fault.

    Why use loop imp for earth fault but PSCC for short circuit? Aren't we doing the same thing for both faults?

    By taking Zs and comparing it to max Zs for the OCPD we are making sure there will be enough fault current.

    We would already know the max Zs so why not just take a second tests after Zs at the point of test Z l-n?


    It wouldn't matter if the circuit had a reduced CPC size as you would be taking both readings. It's also dependent on the PEFC and PSCC at the origin to begin with so taking both impedance readings will give you the info that is needed.


    Farmboy. FYI for some reason your second post did not come up here. The only reason I know you posted again was the email I got.
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    Sparkymania:

    If the L-N loop imp. is too high won't that mean the current flow will then be too low, the same as it would be for L-E loop imp.




    Not necessarily - there is no fixed disconnection time for an L-N fault in a TN-C-S, TN-S or TT system. The requirement is to disconnect before the cable is damaged. So, the requirement is NOT the loop impedance, but the Adiabatic Criterion (or, for disconnection times longer than 5 s, calculated in accordance with BS 7454) - see Regulation 434.5.2.

     




    We don't test for PEFC to make sure an MCB will trip on an earth fault.




    That's because we need disconnection time in accordance with Chapter 41 - and therefore we check loop impedance in accordance with the relevant Table 41.2 to 41.5 (adjusted by the "rule of thumb" of course, as we are taking measurements at 20 deg C, not the operating temperature of the cable 70 deg C).




    Why use loop imp for earth fault but PSCC for short circuit? Aren't we doing the same thing for both faults?




    The prospective fault current at the start of the circuit is greater than the prospective fault current at the end of the circuit, and hence this is really the reason we don't often do pfc checks at the end of the circuit, because usually the thing we are worried about for damage to the cable is worse at the DB.




    By taking Zs and comparing it to max Zs for the OCPD we are making sure there will be enough fault current.

    We would already know the max Zs so why not just take a second tests after Zs at the point of test Z l-n?




    You could to that, but then you'd have to work out what the current is to make use of the adiabatic criterion, or BS 7454, so it's easier to measure the current.




    It wouldn't matter if the circuit had a reduced CPC size as you would be taking both readings. It's also dependent on the PEFC and PSCC at the origin to begin with so taking both impedance readings will give you the info that is needed.




    Not necessarily. But I agree it's not really necessary to take the measurement if the cable is sized correctly in accordance with BS 7671 - the problem for L-N and L-L faults as I said happens further up the cable, not further down.


    Volt drop is usually a limiting factor if you do the calculations, and have selected the cable in accordance with BS 7671.






    Farmboy. FYI for some reason your second post did not come up here. The only reason I know you posted again was the email I got. 




    Apologies - I can see both but I wonder whether it's because it's gone "over the page"? Not sure.

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    Thanks for the replies everyone.


    gkenyon. The last line in my comments was to Farmboy not to you. It was his post that didn't appear. Yours did. (You must of wondered what I meant by Farmboy. I wasn't calling you a farm boy, ha ha).


    Good points. It's hard to get it into my head because I was never taught this and no one I know does this. The forms only have Zs for each circuit.

    Sparkyninja's recent videos bring out the point that the standard test result sheet are not sufficient. 

    I've heard of the adiabatic equation of course, but have never used it. I don't know anyone who has. 


    Still, you've given me something to think about. The next time I test I will have this in mind, especially when I get a Zs reading higher than max Zs as that will give an indication that the line-neutral may have an issue as, especially if the CPC is the same size.

    With a little practice it should become clearer.



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    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.
  • 0

    davezawadi:

    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.




    Not with mcb's, because of the threshold between thermal and magnetic operation. Having said that, in general an mcb will perform better than the worst-case curves shown in BS 7671.



     

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