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Topic Title: TT submain protection
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Created On: 17 May 2013 02:23 PM
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 17 May 2013 02:23 PM
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anastasis

Posts: 583
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Hi,

Client needs a couple of 60A single phase submains off a three phase TT supply. Currently only one phase is in use feeding a local DB with RCD incomer, so no overall three phase RCD is currently installed.

I can obviously use the two spare phases, and I think the best solution for each submain is a 2 pole 100mA type S (time delayed) RCD and a SPN switchfuse.

I can't think of a better/neater approach, but I don't do much TT stuff - are there any alternatives?
 17 May 2013 07:29 PM
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MobileFireUnits

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I don't have an alternative, but the one you suggested sounds like it would suffice! I hope you get it done, and it works out best for you!
 17 May 2013 08:50 PM
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GB

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Schneider do a range of mccbs c/w time delay Rcd options which may be neater but may also be more expensive!!
 17 May 2013 08:58 PM
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anastasis

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GB, thanks, that sounds interesting although, as you suggest, I suspect the cost will be prohibitive, especially for this client. Do you have any part numbers to save me trying to find them?

I was wondering if I could assemble something from an RCD and DIN rail mounting HRC fuse holder. I can't see why the RCD couldn't double as an isolator. But I haven't been able to find a fuseholder - any suggestions for a 60A DIN rail mounting one?
 17 May 2013 09:02 PM
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anastasis

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Just to add that my original idea was a Wylex WRMT100/2 in an ESI2S enclosure, feeding a Wylex 160CM switchfuse. It doesn't have to be Wylex but they're cheap and readily available.
 20 May 2013 12:05 PM
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AJJewsbury

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But I haven't been able to find a fuseholder - any suggestions for a 60A DIN rail mounting one?

Try: http://www.meteorelectrical.co...units_switchfuses.html (neozed style, max 63A)

or http://www.mdelectricaldistrib...s-Inc/flypage.tpl.html (BS 1361/BS 88), takes 60/63A, 80A or 100A fuses.

- Andy.
 20 May 2013 03:39 PM
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anastasis

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Thanks Andy, having thought about it I'm minded to go for the separate 100mA RCD and switchfuse combination. Now, to get my head round the DBs on the ends of the submains... dual RCD or RCBOs? There seems to have been quite a bit of discussion about this, to put it mildly.
 21 May 2013 01:28 PM
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AJJewsbury

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dual RCD or RCBOs?

Pros and cons both ways:

Twin RCCBs: DP switching so N-PE faults in final circuits are isolated so allowing the submain RCD to hold. On the other hand, any earth fault on one circuit will take out other circuits on the same RCCB. Leakage current is limited to (say) two lots of 30mA, so 100mA+ submain RCD shouldn't trip due to accumulated leakage in final circuits as one (or both) of the local RCCBs should go first.

Single pole switching RCBOs (the usual kind): L-PE faults confined to just the faulty circuit, but N-PE faults will take out the submain's RCD. Cumulative leakage currents could (theoretically) reach 30mA on each way - so if there were 4+ ways it might be possible for the submain RCD to trip first on excessive leakage.

DP switching RCBOs (if you can get them) - both L-PE and N-PE faults confined to the final circuit concerned, but still a chance of cumulative leakage current taking out the submain RCD first (but in the same way that the submain overload protection might go before any of the final circuit overload if diversity isn't quite as calculated).

- Andy.
 21 May 2013 01:50 PM
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marclambert

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Originally posted by: AJJewsbury

dual RCD or RCBOs?


Pros and cons both ways:







Single pole switching RCBOs (the usual kind): L-PE faults confined to just the faulty circuit, but N-PE faults will take out the submain's RCD.

Whilst I appreciate most RCBOs have a solid neutral connection they do still detect N-PE faults don't they?
The discrimination provided by the s type RCD should prevent the Submain RCD from operating just as in a L-PE fault.

Regards
Marc
 21 May 2013 01:54 PM
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anastasis

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Thanks, Andy, but I am fully aware of all this. Deciding which is best is the issue.

With the RCD/RCBO combination, I would add is that is that an L-PE fault is likely to also be an N-PE fault of sufficiently low impedance to trip out the overall RCD. So I'm really not convinced about the usefulness of common-or-garden single pole RCBOs. But there's also nuisance tripping to consider - at least with an RCBO it would only take out one circuit, rather than a group.

On the subject of double pole RCBOs, are there any recommendations for firms that make them, along with suitable busbars, main switches, and enclosures? Of course they'd need to be readily available and reasonably priced. Do I see a pig flying past my window?
 21 May 2013 02:11 PM
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anastasis

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Originally posted by: marclambert
Whilst I appreciate most RCBOs have a solid neutral connection they do still detect N-PE faults don't they?

The discrimination provided by the s type RCD should prevent the Submain RCD from operating just as in a L-PE fault.


Regards

Marc


Yes, a single-pole RCBO will detect and trip on an N-PE fault. But it won't isolate the fault as it only disconnects the live. You'll still have an N-E connection in circuit which will present a alternative path for the neutral current. This will divert current (from other loads) to earth and if there's sufficient imbalance through the overall (type S) RCD, it will trip.

(There's a few complications to consider which I don't have time to go into, but that's what happens in most cases)
 21 May 2013 02:21 PM
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marclambert

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Fair enough.
I read Andy's post to read that the RCBO would not operate. That was the basis for my response.
Regards
Marc
 21 May 2013 02:21 PM
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AJJewsbury

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Whilst I appreciate most RCBOs have a solid neutral connection they do still detect N-PE faults don't they?
The discrimination provided by the s type RCD should prevent the Submain RCD from operating just as in a L-PE fault.

Yes, they still trip, but leave N (and PE) connected, so even after expiry of the submain RCD's time delay, the fault is still there and so the RCD should then trip.

On the subject of double pole RCBOs, are there any recommendations for firms that make them, along with suitable busbars, main switches, and enclosures? Of course they'd need to be readily available and reasonably priced. Do I see a pig flying past my window?

You could try the Garo range from Meteor - http://www.meteorelectrical.co...cbo-s-mcb-s-rcd-s.html (go up a level for DP bus bars, enclosures etc) - I've bought a few of the 2-module ones and they seem OK. Unfortunately they don't seem to do a DP isolator in the same range, although the Proguard one seems to fit if you use 'finger' type busbars (the terminals don't fit the Garo 'fork' type bus-bars).

- Andy.
 21 May 2013 02:24 PM
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AJJewsbury

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I read Andy's post to read that the RCBO would not operate

Ah, yes I can see how you could read it that way now - not well written! - apologies for that.
- Andy.
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