rcd requirements on a tt system

[Engineer54]

I totally understand what you're getting at but technically speaking, you're wrong.

An RCD of any size on a TT provides fault protection, a 30mA rating will provide additional protection.

If an upfront type S was fitted, I'd call that back up protection

We'll agree to disagree, on this one, it'll be easier all round!! lol!!

 

[NickD]

So it can't really be classed as Additional protection can it

It's additional protection over and above basic protection, i.e. insulation of live parts (mostly).

 

[NickD]

Cables buried in walls less than 50mm require additional protection by means of a 30mA RCD, therefore a 100mA RCD would not be sufficient to protect it.

Indeed. Sorry, I'd skimmed over the fact we were talking about a 100mA upfront. (Yes I do know about discrimination in operation of RCDs in series, before anyone reaches for the flamethrower.)

 

[Engineer54]

It's additional protection over and above basic protection, i.e. insulation of live parts (mostly).

Which is where your normal Zs values would come into play on and RCD failure!!, but unfortunately not on your typical UK TT System. So it's not additional, it's the sole means of earth fault protection!! A 100mA S type would be the additional, (or if you like back-up) protection!!

 

[D Skelton]

Eng mate! 415.1.1!

It's wrong I know, that you can expect an RCD to provide both fault protection and additional protection but they can! lol

A type-S up front, although highly advised, would still only be supplementary to the means of fault and additional protection.

 

[Engineer54]

Fortunately or unfortunately, i prefer to live in the real world rather than what BS7671 says!! On a UK TT system, If you lose the 30mA RCD operation for any unforeseen reason, that's your installations (or part of) earth fault protection gone!! There is nothing additional about them, only good while they are working....

In the vast majority of UK TT situations, the up-front 100/300 mA S type won't exist!!

 

[D Skelton]

Fortunately or unfortunately, i prefer to live in the real world rather than what BS7671 says!!

As do I. When the question regarding back up protection by way of a type-S up front comes up I think you'll find we have little to disagree on! I also agree that in parts the BGB spouts utter nonsense, however all I have done is provide answers to MINIMUM requirements and certain definitions.

Nonsense or not, the BGB is all we HAVE to follow but if you were to ask me how I would personally do it I would be on your side the whole way! (Apart from the whole 'effective Ra value' debate )

 

[Engineer54]

As do I. When the question regarding back up protection by way of a type-S up front comes up I think you'll find we have little to disagree on! I also agree that in parts the BGB spouts utter nonsense, however all I have done is provide answers to MINIMUM requirements and certain definitions.

Nonsense or not, the BGB is all we HAVE to follow but if you were to ask me how I would personally do it I would be on your side the whole way! (Apart from the whole 'effective Ra value' debate )

With a little effort at times, you may well be surprised at the results you can achieve. It's the do nothing attitude that pee's me off!! lol!!