rcd requirements on a tt system

[ambrad]

Hi all

A new shower circuit has been installed into a town property that is a TT earthing set up. The shower is protected by a 60amp 30 ma rcd and a 40 amp breaker. The ZS is 9.8 ohms, the Ze is 9.7 oms. Should this circuit be protected by a further 100ma rcd before the 30ma rcd or is the 30ma rcd adequate.

regards

 

[telectrix]

the 30mA RCD is adequate to protect the shower circuit. however, a 100mA S type up front is a good idea on a TT system to achieve disconnection times for the installation as a whole.

 

[NickD]

(Warning: me newbie). The upfront RCD will also protect the cable between it and the shower DB with the RCD+MCB in it (I assume that's what you're talking about) which may be a requirement e.g. if the shower cable is buried in a wall.

 

[ambrad]

the 10mm cable is in trunking at ceiling height from the db near the front door to the 1st floor bathroom. no other circuits are on the db.

 

[NickD]

Where's the RCD and breaker for the shower located?

- - - Updated - - -

Or do you mean that's what's by the front door?

 

[ambrad]

yes s you enter the property, in the hall of a street house

 

[Henryk]

Hi guys do you have to allow for surge protection now on over head mains now?
Just a thought

 

[D Skelton]

The 30mA RCD is more than adequate in providing the required disconnection time for the shower circuit. It also provides additional protection as per the requirements of section 7 in the BGB regarding special locations therefore you do not need to install an upfront type S but in many cases it would be prudent to do so.

(Warning: me newbie). The upfront RCD will also protect the cable between it and the shower DB with the RCD+MCB in it (I assume that's what you're talking about) which may be a requirement e.g. if the shower cable is buried in a wall.

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.

 

[Engineer54]

The 30mA RCD is more than adequate in providing the required disconnection time for the shower circuit.

It also provides additional protection as per the requirements of section 7 in the BGB regarding special locations therefore you do not need to install an upfront type S but in many cases it would be prudent to do so.

QUOTE]

Not on a TT earthing system it doesn't, it's the ONLY means of earth fault protection!! And why having a 100mA S type RCD up-front, is a prudent additional/back up protection. This mainly due, because RCD's as a whole, are not particularly known for their reliability as yet!!

 

[D Skelton]

I don't understand what point you disagree with Eng? A 30mA RCD does provide additional protection regardless of what the nature of the supply is, even if it is the sole means of providing earth fault protection also.

 

[Engineer54]

I don't understand what point you disagree with Eng? A 30mA RCD does provide additional protection regardless of what the nature of the supply is, even if it is the sole means of providing earth fault protection also.

ADS is the primary earth fault protection on TN earthing systems, with 30mA RCD protection providing the ''Additional'' protection. On a TT system however, the ADS is basically null and void and in most cases, unable to provide any form of earth fault protection!! The 30mA RCD is then providing the sole means of earth fault protection. Which is all fine and dandy while the RCD is functioning correctly. So it can't really be classed as Additional protection can it, it could i suppose be classed as an enhanced form protection... lol!!

 

[D Skelton]

It can be both. It is providing both fault protection and additional protection. As I said in my first reply, the OP doesn't need to fit an upfront RCD but it would be prudent for him to do so, that way, if the 30mA fails, yes the customer may lose their additional protection but at least they still have their fault protection. If he didn't, he wouldn't be breaking any actual regulations.

I was merely satating the facts and in no way advising him. Anyway, we're arguing the same fundamental point regarding it being a good idea to fit an upfront type S lol.

 

[Deleted member 9648]

ADS is the primary earth fault protection on TN earthing systems, with 30mA RCD protection providing the ''Additional'' protection. On a TT system however, the ADS is basically null and void and in most cases, unable to provide any form of earth fault protection!! The 30mA RCD is then providing the sole means of earth fault protection. Which is all fine and dandy while the RCD is functioning correctly. So it can't really be classed as Additional protection can it, it could i suppose be classed as an enhanced form protection... lol!!

How do you arrive at the conclusion that ADS doesnt apply to a TT system???.....ADS is the standard protection against electric shock on all the commonly used systems in the UK,including TT,where ADS is usually achieved by nmeans of an RCD.

 

[Engineer54]

By ADS perhaps i should of stated conventional means of earthing!! RCD protection is classed as ''additional'' protection on TN systems. It sure ain't additional protection on a UK TT earthing system is it?? It's RCD or it's nothing!! ...lol!!

 

[D Skelton]

It sure ain't additional protection on a UK TT earthing system is it?

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

 

[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!!