Rcbo on a tt system

[ProDave]

but surely it must be isolated under fault conditions?

So a TT system needs double pole mcb's as well then?
 

 

[Tony S]

but surely it must be isolated under fault conditions?

So a TT system needs double pole mcb's as well then?
 

Some manufactures specify SP RCBO’s unsuitable for TT systems as they don’t isolate a N→E fault.

 

[steptoe]

So a TT system needs double pole mcb's as well then?
 

no, an MCB trips on overcurrent,

an RCD trips on E fault

 

[Tony S]

Sorry Dave I misread your post, SP MCB’s are OK.

Its SP RCBO’s that are the problem as they don’t clear a N→E fault. An up front RCD to clear N→E faults would make RCBO’s pointless.

 

[steptoe]

Sorry Dave I misread your post, SP MCB’s are OK.

Its SP RCBO’s that are the problem as they don’t clear a N→E fault. An up front RCD to clear N→E faults would make RCBO’s pointless.

now,

that depends,

30mA SP RCBOs and an upfront TD100mA  :C

 

[Tony S]

now,

that depends,

30mA SP RCBOs and an upfront TD100mA  :C

A SP RCBO will clear a L→E fault but not a N→E fault. The DP TD RCD would clear the fault at the same time blacking the entire house out, which circumvents the point of RCBO’s.

I’ve seen 45A flowing through a N→E fault where a combined high resistance neutral and a N→E fault existed. It was pure chance bad luck I found it.

 

[steptoe]

A SP RCBO will clear a L→N fault but not a N→E fault. The DP TD RCD would clear the fault at the same time blacking the entire house out, which circumvents the point of RCBO’s.

I’ve seen 45A flowing through a N→E fault where a combined high resistance neutral and a N→E fault existed. It was pure chance bad luck I found it.

Exactly, that's why I don't see any point in SP RCBOs, 

 

[Sidewinder]

IMHO, every electrician installing, modifying, repairing or fault finding should have their own test equipment to ensure that their work is safe and compliant.

I disagree with companies that run the idea of testing teams, that go around after the installers have been around, to do the testing, after the installation has been energised.

That is the dead testing to prove the install is safe to energise is done, after the installation has been in use by the householder for at least overnight, if not longer before the testing teams turn up.

RCBO's.

I don't like the idea of SPSN RCBO's on TT, but, an all RCBO install on a TT is an excellent way of splitting up the installation such that the inconvenience in the event of a single fault does not trip the up-front TD RCD (if it's there) for example.

This is why SBS Dave's product is so good.

I couldn't remember the other night if it is a requirement of the regulations to disconnect all live conductors in the event of a fault on a TT which was different to a TN system.

I couldn't find one.

Yes, isolation of all live (or unearthed live) conductors, so on TN you can legitimately isolate only the Line, IF, the N is solidly earthed.

Allowing for there being no chance of any "back feeding" of the N from elsewhere.

That aside, on a TT as the N is not strictly referenced to the local earth, then this "has" to be isolated when isolation is required.

However, I could not find a reg that required disconnection of the N in the event of a fault on a TT.

So, can anyone come up with the reg no please?

I was convinced that it was there, but, I couldn't find it.

Please humour me as I don't have a regs book any more & I ain't buying one that's for sure, I've been conned out of enough money for the so called "online" regs!

I can't find the requirement for disconnection of N or all live conductors in the event of a fault on a TT system.

I would love to be able to, but, failed.

 

[Risteard]

However, I could not find a reg that required disconnection of the N in the event of a fault on a TT.

So, can anyone come up with the reg no please?

I was convinced that it was there, but, I couldn't find it.

There isn't one.

Also, you stated that only line conductors (as opposed to live conductors) need to be isolated in a TN installation. However this is not the case for an installation not under supervision by skilled or instructed persons (or, more accurately, whatever the new term is for these classes of people) and therefore is not the case in any domestic installation. In any domestic installation ALL live conductors must be broken for isolation.

 

[Sidewinder]

Risteard,  it wasn't a full in depth explanation, it was a can someone just remind me please, because I couldn't remember, and couldn't find a reg.

I believe that you are referring to 537.1.4, where as I was referring to 537.1.2.

So, we are both correct in that aspect.

It seems they call them "ordinary persons" this week, and it seems to apply to the main switch at the origin.

Which is normal anyway.

I was just convinced in the back of my mind that there was, something different with single pole fault protection as applied to TT over TN.

I did several searches, and could not find it.

I note that Schenider state that their devices are not suitable for IT/TT systems though, as these systems "require double pole switching".

I remember looking for this when I read that for the first time, and failing to find it then, another question came up recently so I looked again, and failed to find it.

I can't see it having been taken out of EN 60364 when it is transcribed into BS7671, so there must be some reason for it.

Having just checked, I am also quite surprised that the 3 new models of Schneider RCBO all have a maximum earth resistance of 100 Ohms stated...

So, 134.1, 510.3?...

 

[binky]

. In any domestic installation ALL live conductors must be broken for isolation.

But isn't that just the main switch?

 

[Risteard]

But isn't that just the main switch?

Yes it is. I thought I made that clear when I stated that there is no Regulation requiring all pole disconnection in a fault. Apologies if that wasn't clear.

 

[binky]

Just wanted to clarify amoungst all the discussion on RCBOs

 

[Tony S]

It’s just another example of an ill thought out regulation. At least Schneider warn about the unsuitability of SP RCBO’s on TT systems.

I don’t like them full stop, they only do half a job. It’s 50/50 if a nail through a cable hits L&E or N&E.

 

[Andy™]

SP or DP makes no difference to me. if you have a fault then it will trip isolating the line, circuit is then dead. everywhere has SP MCB's yet no-one questions them only switching line and not neutral in if tripped due to a fault

 

[Tony S]

The NE fault won’t be cleared allowing neutral current to flow through the fault.

With TT you have to think outside the box (house).

 

[Sidewinder]

SP or DP makes no difference to me. if you have a fault then it will trip isolating the line, circuit is then dead. everywhere has SP MCB's yet no-one questions them only switching line and not neutral in if tripped due to a fault

On TN single pole is fine, the water just gets muddied with TT (& IT, but that's special!)