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Double Pole RCD for TT


Robin Spark

Question

Ok guys or Ladies,

I havent yet worked on TT installs and the ones I have seen have been converted to PME, just trying to work out why we need DP isolation on TT?

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Thanks Specs for all that info, really useful, I will look at these later, hope I didnt keep you up too late last night with you looking through BRB!

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Ok guys or Ladies,

I havent yet worked on TT installs and the ones I have seen have been converted to PME, just trying to work out why we need DP isolation on TT?

Because neutral could well be at a higher potential than earth

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Thanks Specs for all that info, really useful, I will look at these later, hope I didnt keep you up too late last night with you looking through BRB!

you didnt - its all in his memory ready to be typed onto here at a seconds notice

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Oh yeah of course I knew that LOL!

Have you got the red osg Riggy, looking at page16 general layout of equipment (for TT installs) it shows CU with normal main switch and then individual RCBO's which are normally single pole. So this wouldnt comply would it?

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DP Isolation is on the supply. The circuits on a TT system need to be on RCD but not Double Pole.

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Oh yeah of course I knew that LOL!

Have you got the red osg Riggy, looking at page16 general layout of equipment (for TT installs) it shows CU with normal main switch and then individual RCBO's which are normally single pole. So this wouldnt comply would it?

Sorry OSG in van (and it's dark outside) if someone cleverer doesn't answer, I'll have a look tomorrow.

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i think u will be hard pressed to find a reg requiring DP fault protection for TT systems, i could be wrong though...

there is a reg relating to safe isolation and the neutral when it it not effectively connected to earth.

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DP Isolation is on the supply. The circuits on a TT system need to be on RCD but not Double Pole.

So your saying that Double Pole is neded for isolation only and so single pole protective devices are within regs for fault protection.

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Yes, but don't take my word for it, I'm not 100% on TT's yet but you deffinately don't need DP isolation on your final circuits.

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Thanks everyone for your input, just been looking at Regs book and I think 537.2.1.1 confirms this:

Every cct shall be capable of being isolated from each of the live supply conductors. In a TN-S or TNC-S system it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliably connected to earth by a suitably low impedance

So on boards to 16th or previous, you would have 100ma time delay as main switch and that would have provided the fire protection to all ccts, but then in addition a 30 ma rcd for sockets etc.

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Thanks everyone for your input, just been looking at Regs book and I think 537.2.1.1 confirms this:

Every cct shall be capable of being isolated from each of the live supply conductors. In a TN-S or TNC-S system it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliably connected to earth by a suitably low impedance

So on boards to 16th or previous, you would have 100ma time delay as main switch and that would have provided the fire protection to all ccts, but then in addition a 30 ma rcd for sockets etc.

You are not quite right on this reg cuz that is NOT applying to TT.

however.. some of the bits you would be interested in are...

Reg 531.2.1 page 112:

An RCD MUST disconnect all LINE conductors at near enough the same instant!

531.4.1 page 113:

If a TT installation is protected by a single RCD it shall be placed at the origin.

411.5.2 & 411.5.3 page 50:

Fault protection can be provided by either RCD or overcurrent protection device.

where low Zs cannot be met RCD's preferred option.

Page 16 of OSG is part of the "general layout of equipment" section...

more to do with Meter/Bonding & earth arrangements etc.. IMHO!;)

Pages 24 / 25 have the example of compliant RCBO installations TN & TT!

The reg 531.4.1 page 113: single RCD TT installations would not generally meet the reg 314.1. (iii) page 39, in your average house insatlltion...

BUT if it was a small garage or outbuilding that had a TT rod installed and a small 2-way CU with light & socket circuit.. a single RCD can be considered suitable to protect the garage/shed installation in which case this MUST disconnect BOTH conductors because it is ALSO the isolator switch for that part of the installation. and needs to disconnect ALL 'live' conductors (inc neutral) 537.2.2.1

But a house with multiple RCBO's doesn't need to diss 'L' & 'N'

:D

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so all SP RBCOs are non-complaint then according to 531.2.1? curious now

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so all SP RBCOs are non-complaint then according to 531.2.1? curious now

TT installations requires double pole isolation but this can be done with main switch. You can use single pole Rcbo's in board but not on some installations being caravan parks, jetties are another I think and there are a few others. But I may be wrong when I find my reg book I will look that reg number up.

Batty

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cheers batty,

i was clear enough on isolation, i was wondering more about if specs post refered to in particular 531.2.1 was refering to fault protection?

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There are TT installations that require isolation of L and N these being caravan parks and some others too idle to look them up, and that means double pole RCD's.

Batty

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