TT submain

[ianphilip007]

hi guys

Need a little help, not done much work on TT supply systems. Have a job lined up to supply an outbuilding from the main house TT supply. Customer wants a seperate sub main to supply the outbuilding. plan on running the submain as per usual ( split tails via henly then into switch fuse(metal clad) then to outbuilding using SWA. i intend to protect the submain via a 100ma type s RCD. now comes the problem, where to place the RCD protection for the submain. is it best to place it on the incoming tails before the henly block, or between the henly block and the switchfuse for the submain. note final circuits in outbuilding will have 30ma rcd proctection.

any help appreciated.

 

[jaresquire]

Is there a particular need for this RCD?

 

[pushrod]

. Are you putting in another earth rod at the outbuilding?

Edit - apologies first part of post deleted - had not read main post properly!

 

[johnboy6083]

every circuit on a normal TT system will require an RCD to meet disconnection times. the only exceptions to this are stable low readings, but are probably outside the scope of most domestic installations.

i would henley block to 100ma s type RCD, rcd to switchfuse. from switchfuse to outbuilding in SWA, new board in outbuilding protected by 30ma RCD.

 

[ianphilip007]

hi thanks for your responses.

i thought rcd protection was required on a TT submain to provide fault protection, as if the cable is damaged then the possibe fault current generated would not be enough to trip or blow the protection device. ( does the use of SWA negate this )

i also thought that if the TT submain tails were to enter a metal enclosure ie switch fuse or C.U. the submain tails have to be RCD protected.

i chose the 100ma stype RCD for the submain protection to allow the final circuits in the outbuilding to be protected via 30ma RCD. discrimination taken into account.

as i aid expierience on tt limited so my understanding may be wrong.

 

[johnboy6083]

what you said is correct.

the use of SWA does not negate this, as it cant increase the fault current, in order to improve the discinnection times.

 

[ianphilip007]

as per extra earth rod dependent on ra reading but i might install extra rod yes ,

 

[johnboy6083]

more earth rods are a good idea IMO.

 

[topquark]

NB We're all assuming that the overall current requirement is below the main fuse rating as you don't mention the power requirements.

 

[ianphilip007]

yes main fuse is 80amps, sub main fused via switchfuse (40A) . So Sub main has overcurrent protection for the cable and main head fuse is protected, but as TT system the fault protection for sub main is provided by RCD 100ma type S due to high ZS.

is it ok to export the Earth from the house Earth rod ?

 

[jaresquire]

Didn't spot that the house was TT so your proposal is spot on. As Johnboy says, the more rods the better, at least one at outbuilding.

 

[topquark]

As you're using a type S device at the feed end of the dist circuit, I'd be wanting an earth rod(s) for the outbuilding as none of the safety devices would work if you lost the exported earth.

 

[ianphilip007]

many thanks for all your replys most helpfull, glad to know i had the right idea

 

[Des 56]

hi thanks for your responses.

i

i also thought that if the TT submain tails were to enter a metal enclosure ie switch fuse or C.U. the submain tails have to be RCD protected.

.

I dont think the tails need Rcd protection, if they are protected at the entry of the metal clad consumer or switch by a plastic cable gland or simillar

 

[yellowvanman]

Where you have tails going into a metal clead enclosure there could be a problem if the tails got damaged and shorted to the enclosure, in which case what would trip, nothing!

So the 1st enclosure should be Class II (ie. insulated) - see 531.4.1

 

[Des 56]

Where you have tails going into a metal clead enclosure there could be a problem if the tails got damaged and shorted to the enclosure, in which case what would trip, nothing!

So the 1st enclosure should be Class II (ie. insulated) - see 531.4.1

Not as far as the IET are concerned,I will try and dig out the guidance when I get chance

 

[yellowvanman]

I've quoted the reg number from the 17th which is from the IET, although my 1st paragraph is my description.

Look at it this way - if there is a fault to earth on the supply side of the 1st RCD nothing will trip - if the enclosure is Class II then problem is limited and whilst there maybe a fault an accessible enclosure doesn't become live and stay live.

 

[Des 56]

I dont dis agree with your reasoning,it is sound

The option of using a suitable insulated entry gland is definitely an IET decision

It reduces the chance of contact of those tails with the metal casing and has been permitted, and is a recomended method by the IET ofconnecting the tails to a metal clad board without resorting to the use of an Rcd

I just found this esteemed mans post on the IET forum on this issue
Bold and underlined by me
Quote

This problem came to the fore in the late 80's. In one case a contractor rewired a large number of council houses and metal clad consumer units. He had assumed a supply earth but the state of the local network did not allow it and TT had to be used.

Each unit was fitted with a front end 30mA RCD but this left the problem of the risk from faults between the metal case and the incoming tails. The consumer unit manufacturer came up with the 'insulating kit' solution and this was later adopted by others.

The NICEIC were actively involved in the discussions that took place.

The 16th Edition introduced a regulation to normalise the situation [531-04-01] in 1991 when it was first issued. This regulation is still in use [531.4.1]. It allows a metal enclosure to be used provided that the part of the installation between the origin and the input terminals of the RCD meet the requirements for class II. In the manufacturer's view, this can be achieved by improving the bushing around the cable entry, using PVC/PVC tails, and securing those tails. All of this does indeed reduce the probability of a fault between the tails and the metal case.