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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

    According to the book the time-current table for a 60898 B, C or D 32 would allow a continuous current of 46A and not trip.



    For most devices they're required to trip within 'conventional time' (usually one hour for domestic size devices) at 1.45x In - smaller overcurrents will still trip eventually but will of course take longer. After all that's the whole purpose of the thermal element of the MCB.


     

    I'm wondering how the cable would cope with that.



    Surprisingly well actually. Cables don't suddenly explode or catch fire the moment their rating is exceeded. Generally they just run a little bit warmer with the result that the insulation degrades a little faster than usual. Given that a PVC cable might have a 70 years life span even if a fault or two knocks a few months or even years off a cable's life, is not all that significant. We accept 70 degree PVC cables getting to 160 degrees during high current faults - it will take a cable a long time to get to that sort of temperature with what's the equivalent of only about a 50% overload.


    From the point of view of needing to check that the MCB/RCBO will clear faults instantaneously (i.e. verifying that the fault current exceeds say 5x In for a B-type MCB) - that's really only needed where the cable is rated lower than the protective device (e.g. industrial motor circuits or ring final circuits) - in the majority of cases where the cables are rated the same or higher than the MCB/RCBO then they're automatically protected from low current fault currents in just the same way as they would be from overloads.


       - Andy.
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  • 0

    According to the book the time-current table for a 60898 B, C or D 32 would allow a continuous current of 46A and not trip.



    For most devices they're required to trip within 'conventional time' (usually one hour for domestic size devices) at 1.45x In - smaller overcurrents will still trip eventually but will of course take longer. After all that's the whole purpose of the thermal element of the MCB.


     

    I'm wondering how the cable would cope with that.



    Surprisingly well actually. Cables don't suddenly explode or catch fire the moment their rating is exceeded. Generally they just run a little bit warmer with the result that the insulation degrades a little faster than usual. Given that a PVC cable might have a 70 years life span even if a fault or two knocks a few months or even years off a cable's life, is not all that significant. We accept 70 degree PVC cables getting to 160 degrees during high current faults - it will take a cable a long time to get to that sort of temperature with what's the equivalent of only about a 50% overload.


    From the point of view of needing to check that the MCB/RCBO will clear faults instantaneously (i.e. verifying that the fault current exceeds say 5x In for a B-type MCB) - that's really only needed where the cable is rated lower than the protective device (e.g. industrial motor circuits or ring final circuits) - in the majority of cases where the cables are rated the same or higher than the MCB/RCBO then they're automatically protected from low current fault currents in just the same way as they would be from overloads.


       - Andy.
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