Consumer Unit protected by 500mA trip

[JGH1948]

I'm changing the CU in an elderly lady's late 1970s flat from an old four way wire fuse board to a 10-way split board with one side protected by an 80mA RCD and want to keep down the costs for her. The old board is currently protected by a separate 500 mA RCD isolator which old but works fine. I can't see why I shouldn't leave that in place can anyone else?

 

[GaryMo]

Don't you mean 80A 30mA RCD for your new split load CU?

 

[JGH1948]

I certainly do. Sorry posting on the run.

 

[RF Lighting]

What type is the earth supply to the property?


Is the existing RCD an ELCB or an RCCB?

There isn't really much point in having a 500mA RCD, and it is unlikely to be time delayed, so you will not have discrimination between this and your new 30mA RCD which contravenes BS7671.

If the supply is a TT, your 500mA device has a rating too high for a Ze above 100 ohms

 

[JohnD]

sounds like someone stole it from a factory

 

[ricicle]

If the supply is a TT, the maximum rating allowed is 100mA, so your 500mA device has a rating too high.

Although it's standard practice to fit the 100mA (TD) as the largest rated RCD on a TT it is not actually stated in the regs.As long as the formula RaIa<50V is met.

 

[Adam_151]

If the supply is a TT, the maximum rating allowed is 100mA, so your 500mA device has a rating too high.

Are you sure?

Table 2.3 in GN3 lists the max TT Ze requires for use with devices upto 500ma (based on the RaIa<50 thing)

 

[RF Lighting]

I have edited my post.

 

[Taylortwocities]

JH, what else is written on that "500mA RCD"?
Does it sat Chilton or does it have a big green on button.

Voltage Operated Earth Leakage Breaker and probably a TT installation.

Voltage Operated Earth Leakage Breaker. You can identify it if it has two earth wires going to it. One in and one out. If so this need replacing as they do not work in todays earthing environments.

If its what i think your eldery lady is in for a few more £££s

More info and/or a pic please!

 

[JGH1948]

Thank you all for your advice. I haven't had a chance to check but I don't think its a TT because it's a relatively modern block in a town. I haven't seen the isolator - the lady just read off the rating over the phone. I'll get pix next week when I check it all out myself.

Certainly if it is a TT sytem I would have thought best practice alone means a 100ma RCD protecting the whole system and a further 30mA RCB protecting the socket circuit?

 

[Taylortwocities]

Certainly if it is a TT sytem I would have thought best practice alone means a 100ma RCD protecting the whole system and a further 30mA RCB protecting the socket circuit?

That will be a time delayed 100mA RCD then...