3 Phase TT supply and RCD

[Jim2287]

Hi, have problem

Currently a metal DB with failing RCD installed
3 phase TT supply
Need to change DB for Double insulated IP rated with 100A 100mA TD RCD

Problem is .... a time delay RCD trips with 0.13s-0.5s but under 17th edition, it must trip 0.07secs as i understand from Table 41.1 (Page 46 BS7671)

Table states that the times are for 2x rating of RCD so ... 100mA Time delay trips in 0.06-0.2 secs at 200mA fault ( Page 243 Table 3A)
This is only just on the limit if i happen to get a lucky rcd ...
Most circuits will be single phase on 30mA RCBOs but one is 3phase 16Amp and cant find 3phase rcbo .. the 100mA will cover this

Is there something i dont understand or doing wrong ?

Should i be using design voltage of 230v even though its 240v to earth?
If i said its 230v i can have 0.2 secs disconnection time on TT
any help appreciated
thanks

 

[Spark123]

Do they make double insulated 3 phase boards? Only ones I can remember seeing are metal. To use a metal one means that either an RCD has to be mounted in a plastic enclosure external to the board or some sort of a kit installed in the board to ensure that a fault can't occur pre RCD causing a short to the metalwork.
Think larger circuits and distribution circuits can have up to a 1s disconnection on a TT system.
2x RCD thing was binned with the first corrigendum to BS7671:2008 - guess all the test instrument manufacturers who sold testers as 17th compliant without a 2x test weren't very happy!
Uo is line to earth voltage, the nominal voltage is 230v even on 3 phase.
Not sure if they make RCBOs in 3 phase flavours.
I'd be tempted to install the circuit from the 16A MCB to an external 3 phase RCD, the cable in between the CU and RCD classed as distribution.

 

[ericmark]

vigi rcbo are available three phase they come in two bits and you connect them together and it takes 6 ways but I used loads on T5 project and they worked well.

 

[plugwash]

something else to bear in mind, single module RCBOs don't generally isolate the neutral making them not suitable for TT installs (I'm not sure if there is an expcit reg preventing thier use but at best you have the problem that they won't discriminate with the main RCD in the case of N-E faults)

 

[Jim2287]

Think my boss found a hager double insulated 3 phase board, but if cant find one, like u say external RCD.... seems this would perhaps be best anyway as pointed out by plugwash, RCBOs wont stop a neutral fault...

so maybe external rcd to supply 3x double insulated single phase boards (maybe with their own RCD as mainswitch to discriminate), and just a 16amp 3phase mcb for the 3 phase circuit


can anyone clarify which disconnection time i should be using (I know 3 phase makes no difference as it is voltage to earth)

120v < Uo <= 230v The design voltage is 230v but real is 240v to earth
Disconnect in 0.2 secs TT, 0.4secs TN

or

230v < Uo <= 400v The real voltage is 240v which falls into this class
Disconnect in 0.0.7 secs TT, 0.2secs TN


thanks for help

 

[ban-all-sheds]

Uo is 230V.

 

[ericmark]

The Vigi C120 modules are available in 4 unit width Class A, AC and si at 30mm see 18569, 18578 and 18597 but are 5 units wide from Merlin Gerin so with MCB will take 9 units width it would also need a MGNB* Distributed neutral kit for however many ways the board is and as far as I know only the Merlin Gerin Isobar range has the ability to turn off ways with the sliders and the central neutral connection I don’t know of any other make. And of course the board needs to be more than 4 way.

I viewed at http://www.schneider-electric.co.uk...sf/luAllByID/F498FD374C67B97F8025707B00502AE7 but it may need login to view PDF