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.

 

[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.