TT system & Hottub

[Duane B]

Seen at a hot tub install on a tt system.  Already installed. 

10mm swa straight from c/u off rcbo off a high integrity circuit c/u was a split load wylex jobbie. 

Noticed no 100A 100ma s type main switch at front end. Just a 100A double pole switch off TT feed in.

No local rod at tub. So all wired back to c/u  but does have local maintenance  ip rated isolator adjacent  to tub. 

Rod sat at an Ra of 38   or is this Ze.   Loop at 8.1 from isolator.

So if it's on an rcbo do we now go a max of 1667 ohms or is that only if the front end is protected by 100A 100ma main switch. 

Sealed main fuse so just advised 100A. 

KA reads 6.9

Not done TT so it's a learning curve here not got a tester for s type rcd. 

 

[Andy™]

Zs can be upto 1667 for 30mA. if its 100mA, its 500 ohms. and you dont need a different RCD tester for S type

TT doesnt always need an RCD main switch

 

[Duane B]

isn't it safer if TT systems have a time delayed unit at the front end, aswell as the 30mA protecting the circuit are we at 1 sec disconnection times,  then as we have both do we limit to 500ohms or still at 1667. ?????

 

[Andy™]

it could be safer in that there is a backup RCD, however there is no requirement for it.

as for max Zs, do you know why its 1667 for 30mA and 500 for 100mA?

 

[Duane B]

not done TT so off top of my head,  earth rod is classed as unstable over 200 ohms or near 200 ohms and depending if it was wet day on test so by fitting a 30mA rcd that gives additional protection and allows us to abuse the Level up 1667 ohms

100mA Time delay units are 500 ohms due to touch voltage is it  50/0.1=500  plus wouldn't test on 5x

that's just off top of my head like I said 1st id seen a TT job,

 

[steptoe]

Is the RCBO double pole,?

That would be my first concern,

If it's not DP then it doesn't comply imho. 

 

[Duane B]

Is the RCBO double pole,?

That would be my first concern,

If it's not DP then it doesn't comply imho. 

so if the rcbo wasn't double pole were in fact leaving a neutral intact on the circuit. ????  but if a main s type were there double pole would it comply

 

[steptoe]

Not imo,

Do you understand why 100mA RCDs are fitted,?

The fault protection would need to be DP, 

100mA doesn't provide fault protection.

 

[Manator]

I love earthing, its one of my favourite subjects and one I have spent most time learning about. It is also one of the reasons why I prefer TT above any other earthing system. Time delayed RCD's are to provide protection to buildings, and to allow for discrimination, I can extend circuits above and beyond what the regulations say by utilising earthing. I have also used metal consumer units years before this new amendment.

 

[Duane B]

I was understood to believe that 100mA are fitted to give time for the 30mA devices to operate on A TT system

 

[Duane B]

the rcbo is single pole device on own, so what if that was replaced with an mcb on the normal side protected by the split load rcd as this would be then DP plus an upfront 100mA jobbie

 

[Andy™]

100mA does not offer any discrimination to 30mA, only S type will, however they cannot always be used because they take too long to trip

you can get DP RCBO's

 

[shanky887614]

100mA does not offer any discrimination to 30mA, only S type will, however they cannot always be used because they take too long to trip

you can get DP RCBO's

100ma s type upfront on a split load board complies 

 

[Sidewinder]

Maybe, but, does a split load board comply?

 

[shanky887614]

Maybe, but, does a split load board comply?

why not its a board with two rcd's and space for some rcbo's

 

[Sidewinder]

So how does it "minimise inconvenience in the event of a fault" according to reg 314.1(i)?

 

[shanky887614]

So how does it "minimise inconvenience in the event of a fault" according to reg 314.1(i)?

a 100ma s type shouldnt trip in a split load board.

the 30ma should trip in 50ms while the 100ma s type shouldnt trip in less than 200ms if i remember right

 

[Sharpend]

The question is: is a dual RCD board compliant, facts are it never has been since its inception! But that's regs for you!

 

[Sidewinder]

Irrelevant.

If there is an NE fault, they, will, trip, so how does that minimise inconvenience?

It doesn't.

End of.

Sorry.

 

[shanky887614]

Irrelevant.

If there is an NE fault, they, will, trip, so how does that minimise inconvenience?

It doesn't.

End of.

Sorry.

in a split load board there double pole rcd's.

if your using single pole rcbos thats your problem