How do I comply TT system

[stevo]

Hi guys wondering what's the best way to do this got a board change on a TT system 3 bed terrace. So we need to offer an SPD however can't have one in a metal C.U without RCD protection on the Tails because of the single insulation feeding the SPD. So to do that that we need an s type RCD upfront. But we can only use a metal clad enclosure there so the price is rising quite a bit.. Or do I forgo the SPD due to cost and take the Tails directly to the main switch obviously correctly supported and properly glanded this will be for my 1st niceic inspection so want to get it right. Cheers

 

[stevo]

Most likely wont go with upfront rcd due to cost . I would rather not fit the spd to be honest due to the issues it brings up. But if customer wants it I want to make sure its done correctly. Also do I need to worry about the spd type with it being TT. And if so does anyone have a link to a suitible C.U with correct type

 

[stevo]

If you use a board with main switch and RCBOs, then these need to be DP.

Why if isolation is covered by the main switch

 

[pc1966]

These are good, but not cheap, and they offer choices for TT and TN options:

Mains Products - Kingsmill Industries

MAINS SPDs TO BS:EN 62305 Device selection information where External Lightning Protection is installed SPD PRODUCT SELECTOR Interactive...

www.kingsmillindustries.com

But more generally the 1+1 style where you have one SPD element L-N and another from N-E you don't have any single point of failure that would allow a L-N short with all of the danger related to it. There are many cheaper options for that configuration, as it is common in the EU to have TT anyway.

 

[stevethesparks]

Most likely wont go with upfront rcd due to cost . I would rather not fit the spd to be honest due to the issues it brings up. But if customer wants it I want to make sure its done correctly. Also do I need to worry about the spd type with it being TT. And if so does anyone have a link to a suitible C.U with correct type

If you look at the fuseboard prices any additional costs would be minimal. If such costs are making such a dent, with respects you are not charging enough.

 

[Wilko]

because of the single insulation feeding the SPD

Hi - made me think
How about using a bit of 6mm 6181Y meter tail?

 

[stevo]

If you look at the fuseboard prices any additional costs would be minimal. If such costs are making such a dent, with respects you are not charging enough.

it would be in a separate metal clad enclosure and the rcds are about 50 a pop. why pay it if its not necessary

 

[stevethesparks]

I think you are over thinking things a little, use a proper tails gland and consider Wilko's suggestion and forget the separate enclosure.

 

[stevo]

I think you are over thinking things a little, use a proper tails gland and consider Wilko's suggestion and forget the separate enclosure.

ok that probably is the case as this will be my 1st inspection just need to make sure on the correct spd type never really thought about it before they have just gone straight in on tn systems.

 

[stevethesparks]

In addition, rcd's / rcbo's have a tendency to fail when you don't want them to. As with TT they will not be additional protection I would definitely go with an upfront time delayed DP RCD. I do not believe you have to, but that would be my preference.

 

[brianmoooore]

Why if isolation is covered by the main switch

'Cause a N-E fault will trip the delayed RCD.

 

[stevo]

'Cause a N-E fault will trip the delayed RCD.

ahh ok thought you meant generally

 

[stevo]

In addition, rcd's / rcbo's have a tendency to fail when you don't want them to. As with TT they will not be additional protection I would definitely go with an upfront time delayed DP RCD. I do not believe you have to, but that would be my preference.

ahh ok in replacement of the main switch wouldnt this mess with any spd tho as it would be down stream. but yes good shout

 

[stevethesparks]

I've got to be honest, I'm not so sure about an SPD with TT unless you have a pretty good Ra - I will leave that for others to comment for now as off to bed

 

[pc1966]

I've got to be honest, I'm not so sure about an SPD with TT unless you have a pretty good Ra - I will leave that for others to comment for now as off to bed

Most electronics is at risk from the L-N volts, so the (typical) MOV SPD covers that. The N-E will normally have a higher damage voltage, so the GTD is enough protection before cables flash over or class-Y filter capacitors go pop.

If you are TT on the end of a long rural cable you might want to look at the type 1+2 SPD that can divert a lot more energy, much as you would if the building has lightning protection. Usually though in the UK the flash density if fairly low.

 

[SJD]

Most likely wont go with upfront rcd due to cost . I would rather not fit the spd to be honest due to the issues it brings up. But if customer wants it I want to make sure its done correctly. Also do I need to worry about the spd type with it being TT. And if so does anyone have a link to a suitible C.U with correct type

A Fusebox time delay RCD is only about £40. If you are trying to contain the cost for the customer, just fit this out of your own pocket and regard it as "insurance" against NICEIC raising a problem not having one installed during your assessment. Also, the Fusebox SPD now sits after a 32A MCB (originally there was no MCB, but they changed that last year).