RCD in old switchboard

This includes types of overcurrent, Short Circuit and RCD protection
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ElectricalApprentice
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RCD in old switchboard

Post by ElectricalApprentice »

Hi,

The boss is sending me to a job which will consist of upgrading to RCD protection in an old black panel switchboard for two power circuits. Usually I would retrofit an enclosure and use RCBO's however I have been asked to use an RCCB then feed into the original fuse holders and use suitably rated plug in MCB's. Although I'm not a fan of this approach, I cant find anything that says its not permitted. Is there perhaps a clause that I maybe missing stating that it isn't permitted?
AlecK
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Re: RCD in old switchboard

Post by AlecK »

It's a perfectly valid approach.
JamieP
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Re: RCD in old switchboard

Post by JamieP »

What is it that concerns you about this arrangement ElectricalApprentice and makes you not a fan of this approach?
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Re: RCD in old switchboard

Post by ElectricalApprentice »

JamieP

I have been told in that there is a potential that ceramic fuse holder based may not rated for the subcircuit PSCC. Not sure if there’s any truth to it by my thinking is that if I’m already having to install an enclosure, then Installing an RCBO is just as easy.
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Re: RCD in old switchboard

Post by AlecK »

SERFs have a FCI rating if 1.5 kA when new, less if degraded by deposits of copper & carbon.
plug-on MCBs & their bases typically have a FCI rating of 3kA. while most rail-mount gear has a FCI rating minimum 6 kA
But if you measure the PSSC; for most domestic installations it won't be above 1 kA; so unless the old SERFs have been degraded they are still OK to remain in service [ESR 113].

What's "just as easy"rfor the person doing the work is only one consideration. Cost is another. So's performing to the terms of the contract (as agreed between your boss and the customer). We can't just go around doing what we think is a "better" job without regard to other factors.
Maybe disciss it with your boss? - most bosses like it when trainees take an interest
ElectricalApprentice
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Re: RCD in old switchboard

Post by ElectricalApprentice »

Thanks for the info AlecK.

The tradesmen at my work have only done new builds so they are unable to answer my questions regarding older installations.

Just one more quick question regarding older imperial size cables, is there a way to tell the size of them as I was unable to find any marking on the cable stove circuit cable itself? The cable supplying the stove circuit was smaller than 6mm but larger than 4mm. The existing fuse holder is the larger range type and the fuse carrier rated to 30A. I was looking at appendix I to determine what would be the appropriate size MCB to replace it with should I have to do one in the future but obviously I need to know the cable size to begin with. It's a 70ies house, typical black switchboard with 1 x range, 4 x power, 2 x light and HWC circuits.
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DougP
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Re: RCD in old switchboard

Post by DougP »

There was only the two sizes used in NZ AFAIK.
7/.036 which was equivalent to 4mm2.
And 7/.044 which was closer to 6.5mm2

If you've looked at the ratings in appendix I, then you'll have a pretty good idea.
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Re: RCD in old switchboard

Post by AlecK »

Appendix I of "3000" has data on metric vs imperial cable sizes
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