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Topic Title: TT Systems and RCBO's
Topic Summary: Problems with a TT system and RCBO's
Created On: 10 October 2018 07:13 PM
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 10 October 2018 07:13 PM
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Cypress

Posts: 2
Joined: 10 October 2018

Hello,

I have a TT installation in a new build house. I decided I wanted to fit RCBO's to every circuit. I have 18 circuits in all.

I'm using single width RCBO's and read on the Schneider instruction sheet they should not be used for TT systems.

So I did more research and decided to fit a 100 A type S RCD (that switches both poles) at the front of the unit. 100mA trip current.

So the idea is the normal 30 mA RCBO's provide normal protection and the type S 100 mA RCD provides overall protection in case internal wires of the CU became shorted.

I have a problem in that the 100 A type S RCD keeps tripping, despite the 30 mA RCBO's not tripping.

Maybe I've wired the neutrals wrong?

Can anyone help.
 11 October 2018 02:39 PM
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ToniSM

Posts: 450
Joined: 21 November 2006

You would be better off using DP RCBO's

-------------------------
The reports of my death are greatly exaggerated.
 11 October 2018 03:10 PM
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geoffsd

Posts: 2299
Joined: 15 June 2010

Is the 100mA RCD in the board or on the tails outside the board?
 11 October 2018 03:50 PM
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mapj1

Posts: 11588
Joined: 22 July 2004

well the S type is seeing different currents in L and N, so presumably there is another path - is perhaps the neutral at the output of the S type earthed somehow, when it shousl not be. (any N-E bond must be supply-side of RCD.)
Does it trip always or only when there is some particuarl lvel of load on the outbound circutis.
Can you verify neutral earth separation with 500v insulation test ?

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regards Mike
 11 October 2018 07:33 PM
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sparkingchip

Posts: 11471
Joined: 18 January 2003

Hi Cypress.

I have sent a suggestion to you to try by private message on this forum.

Andy B.
 12 October 2018 01:11 PM
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AJJewsbury

Posts: 17451
Joined: 13 August 2003

I have a problem in that the 100 A type S RCD keeps tripping, despite the 30 mA RCBO's not tripping.

One possibility is the accumulation of leakage currents - e.g. if you had say 10mA leakage on each of 8 circuits, no individual 30mA RCBO should trip, but the total of 80mA will likely take out the 100mA unit.

- Andy.
 12 October 2018 01:59 PM
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sparkingchip

Posts: 11471
Joined: 18 January 2003

A internet search threw up this discussion on another forum.

I can not see why Schneider are stating the DP devices are required When in amongst other reasons they don't appear to offer a device rated more than 32-amp.

Don't Schneider think that people will want an electric shower in a home with a TT installation?

They seem to be be presenting obstacles to using their products that maybe don't exist.

Andy B.
 13 October 2018 12:47 PM
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sparkingchip

Posts: 11471
Joined: 18 January 2003

Originally posted by: sparkingchip

A internet search threw up this discussion on another forum.



I can not see why Schneider are stating the DP devices are required When in amongst other reasons they don't appear to offer a device rated more than 32-amp.



Don't Schneider think that people will want an electric shower in a home with a TT installation?



They seem to be be presenting obstacles to using their products that maybe don't exist.



Andy B.


The answer to the shower question is no they don't.

If we are going to us European electrical installation equipment we are going to have to think like Europeans.
 13 October 2018 01:40 PM
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sparkingchip

Posts: 11471
Joined: 18 January 2003

And having upmarket domestic appliances from around the world to install requires us to stop thinking that we can can keep wiring homes like we did in the 20th century.
 14 October 2018 01:40 PM
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AJJewsbury

Posts: 17451
Joined: 13 August 2003

If we are going to us European electrical installation equipment we are going to have to think like Europeans.

Tricky - given our typically single phase supplies to domestics we don't really have the option of using a 3-phase 16A circuit instead of a 40 or 50A single phase circuit, like the Germans probably would.

There do seem to be some significantly different approaches within Europe - the French seem to be moving to single phase (max 40A, occasionally 63A) for domestics with the emphasis on limiting maximum demand (so hot water is typically stored rather than instantaneous) and load shedding relays aren't unknown even in domestics. (Probably as a result on their dependence on Nuclear generation). The Germans seem less worried about max demand and seem happier using 3-phase.

- Andy.
 14 October 2018 01:43 PM
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AJJewsbury

Posts: 17451
Joined: 13 August 2003

I agree though, we're not well served in the modular CU component market at the moment - when I did my house I found some very suitable DP RCBOs and enclosures, but matching 100A DP bus bars were practically unobtainable. (In the end I managed with 63A rated ones and a 63A fused submain)
- Andy.
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