Submain on a TT

[South Spark]

If I'm running a submain on a TT system in SWA then do I need to provide RCD protection for the submain? The cable is 4mm with an approx 30m run buried in the ground. Can I get a 32A switched fuse with 30 ma time delay anywhere? Obvs need to provide discrimination with 30ma RCD in the outbuilding.

 

[telectrix]

you don't need RCD protection for the SWA. what you put in the sub-main is protection , if required, for the sub-main circuits.

 

[South Spark]

Even with a TT?

 

[telectrix]

SWA cable does not need RCD protection, no matter what the earthing system is. it's your sub-main circuits that require RCDs.

 

[Amp David]

Still has to meet the disconnection times though, so unless your Ra is low then good luck in that.

 

[telectrix]

as it's a distribution circuit, the dis. time will be 5 secs., so hoefully the fault current will be sufficient ( depending on the Ra, of course ).

 

[Amp David]

Some good Ra value on that job lol

 

[Engineer54]

Some good Ra value on that job lol

Not too difficult to achieve, if you know what you're doing!! The problem in the UK, is this almost total acceptance of the crazy numbty 200 ohms as mentioned in BS7671 and OSG's as being the value to aim for!!

 

[Flanders]

as it's a distribution circuit, the dis. time will be 5 secs., so hoefully the fault current will be sufficient ( depending on the Ra, of course ).

I thought the disconnection time for a TT system for a distribution circuit was 1 secs Reg 411.3.2.4

 

[Irish Mick]

Just a quick thought. 30m with 4mm cable. Has the voltage drop been calculated? Not trying to be awkward.

 

[Amp David]

Not too difficult to achieve, if you know what you're doing!! The problem in the UK, is this almost total acceptance of the crazy numbty 200 ohms as mentioned in BS7671 and OSG's as being the value to aim for!!

Never even mentioned the 200 ohm bull, but in all honesty, do you think that on this job, the value of Ra is going to be low?

I don't somehow

 

[Engineer54]

Never even mentioned the 200 ohm bull, but in all honesty, do you think that on this job, the value of Ra is going to be low?

I don't somehow

lol!! Probably not!!

 

[Engineer54]

I thought the disconnection time for a TT system for a distribution circuit was 1 secs Reg 411.3.2.4

Depends on what sort of TT system you're talking about. Would you apply 411,3.2.4 if talking about a TT ground field with TN Ra values??

 

[Flanders]

Depends on what sort of TT system you're talking about. Would you apply 411,3.2.4 if talking about a TT ground field with TN Ra values??

If that was the case then no 5 secs is fine but not come across a TT system with a low Ra yet. But I'll keep looking

 

[Engineer54]

If that was the case then no 5 secs is fine but not come across a TT system with a low Ra yet. But I'll keep looking

I know of more than just a few domestic TT systems in Essex, that are below 1 ohm, and in some cases, with better than the local towns DNO TN-S values!! Commercial and/or industrial made roded ground fields, are very rarely in excess of 1 ohm... As i said the biggest problem in the UK is this numbty 200 ohm malarkey and using thin twigs for earth electrodes. I'd never even heard of them before coming on UK electrical forums, and still to this day, haven't seen one in the flesh so to speak!! Only relatively recently have i seen a 1.2m 5/8'' rod, i'm used to seeing and using 2.4m and 3.2m rods.... lol!!

 

[AJP]

Just a quick thought. 30m with 4mm cable. Has the voltage drop been calculated? Not trying to be awkward.

That's what I thought!

 

[sparkdog]

I would use a 100 ma S type for the SWA.