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Simonslimline

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Hi..... just a quick question i have regarding RCD on TT systems, would you class the RCD,S as just fault protection, or additional protection aswell?

i am an apprentice doing the proper route, 2357 and was having this discussion today. any help would be greatly appreciated.

Thanks simon.
 
Say it was just a split load board with x2 30ma rcd,s

Then if i had an upfront 100ma s type would that be additional protection?

Thanks simon
 
if your Zs readings are higher thasn the max. values given in BS7671, then the RCD/s are classed as fault protection as well as additional protection. max. Zs for all circuits is then 1667ohms ( assuming a 30mA RCD)
 
Say it was just a split load board with x2 30ma rcd,s

Then if i had an upfront 100ma s type would that be additional protection?

Thanks simon
Very good question M8, I would suggest if the main switch in the DB was a 100 ma S type and then you had a split 30 ma rcd on both sides of the split then it would be additional protection, you have the usual 30ma with a back up 100 ma S type, definitely additional.
 
posted same time. your 30mA RCDs are additional. the 100mA is fault protection. but either or both 30mA RCDs would probably trip first in the event of a fault.
 
yep, it's just ohms law. look at table 41.5 in bgb.
 
Cheers, we dont seem to have gone in to much depth on TT systems at college and i have worked on a few since i started so just want to make sure my knowledge is up to scratch when dealing with this earthing arrangement.

thanks simon
 
On a TT system if you're unable to achieve a TN vaule for your Ra and you are utilising automatic disconnection of supply as the sole protective measure then you must rely on an RCD for fault protection. This can be an RCD of any rating (you are not limited to 30mA) depending on your Ra value and its stability. If however you choose a 30mA RCD to provide that fault protection then it is also providing additional protection because of its 30mA rating. Remember, only a 30mA RCD can be used as additional protection, any other rating (100mA, 300mA etc...) will not provide this.

The decision as to whether or not to provide an up front type-S RCD is one that is completely up to the designer. On a TT system, an up front time delayed RCD will not provide additional protection as they are typically rated at 100mA or more, neither will it provide fault protection for a final circuit as it wil not trip within the required time of 0.2s. A type-S RCD can only be used to provide fault protection to distribution circuits that require a maximum disconnection time of 1s (on a TT system that is) although some people, myself included, like to fit them as 'back up' (a term not defined in BS 7671) protection. In other words, if an RCD fails to operate on one or more final circuits then the type-S will provide disconnection... eventually. Bear in mind however that you are only required to protect against single fault conditions, therefore the installation of an up front time delayed RCD is not always mandatory. In a typical domestic TT installation 99% of the time you will not be required to install a type-S, although like I and others have said, it is highly advised.
 
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On a TT system if you're unable to achieve a TN vaule for your Ra and you are utilising automatic disconnection of supply as the sole protective measure then you must rely on an RCD for fault protection. This can be an RCD of any rating (you are not limited to 30mA) depending on your Ra value and its stability. If however you choose a 30mA RCD to provide that fault protection then it is also providing additional protection because of its 30mA rating. Remember, only a 30mA RCD can be used as additional protection, any other rating (100mA, 300mA etc...) will not provide this.

The decision as to whether or not to provide an up front type-S RCD is one that is completely up to the designer. On a TT system, an up front time delayed RCD will not provide additional protection as they are typically rated at 100mA or more, neither will it provide fault protection for a final circuit as it wil not trip within the required time of 0.2s. A type-S RCD can only be used to provide fault protection to distribution circuits that require a maximum disconnection time of 1s (on a TT system that is) although some people, myself included, like to fit them as 'back up' (a term not defined in BS 7671) protection. In other words, if an RCD fails to operate on one or more final circuits then the type-S will provide disconnection... eventually. Bear in mind however that you are only required to protect against single fault conditions, therefore the installation of an up front time delayed RCD is not always mandatory. In a typical domestic TT installation 99% of the time you will not be required to install a type-S, although like I and others have said, it is highly advised.

Not a lot of people know that...............me included:)
Brilliant mate............great stuff.
 

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