TT systems and RCBO's

[Deleted member 9648]

Why are standard single pole RCBO's not advised for a TT system? Sure I've seen this somewhere....something to do with not disconnecting the N with a N-E fault potentially allowing a fault current to continue to divert to earth from the neutral.. But then a SP RCBO, or MCB on a TN system would result in the same issue. Why only TT's?

 

[jackhammerJIM]

In a TT system there is a large potential difference between N and E .

 

[Deleted member 9648]

In a TT system there is a large potential difference between N and E .

How large?

 

[Risteard]

TT systems are not different in this regard. Isolation of all live conductors will be achieved elsewhere (main switch). However single pole RCBOs are poor for the reasons mentioned (a neutral/earth fault is detected but left in circuit) but this is not unique to TT systems.

 

[Midwest]

Why are standard single pole RCBO's not advised for a TT system? Sure I've seen this somewhere....something to do with not disconnecting the N with a N-E fault potentially allowing a fault current to continue to divert to earth from the neutral.. But then a SP RCBO, or MCB on a TN system would result in the same issue. Why only TT's?

Good question, I was too frightened to ask myself.

 

[jackhammerJIM]

How large?

That would depend on the installations RA .

 

[Deleted member 9648]

Hmmmm.....still don't understand why TT's are singled out. Need a fuller explanation.

 

[Risteard]

Hmmmm.....still don't understand why TT's are singled out. Need a fuller explanation.

They're not. See post #4.

 

[HappyHippyDad]

I too have wondered this. I believe rcbos do meet the requirements of the regulations as they will switch the line and the main switch will be used as the isolator, but this does not answer your question.

The responder (GeoffBlackwell) in this thread (IET Forums - tt new db rcbo question IET forums) seems very knowledgeable and this is a response from him:

There are at least three points to consider here:

1) Most RCBOs are only single pole and will not, therefore, be suitable for isolation on a TT system, as this must be all pole.

Whether this matters or not depends on what you are calling the means of isolation for each circuit. Remember - isolation is required to allow work on conductors that would normally be live.

2) The second point concerns what method of shock protection is employed between the outgoing terminals of the main switch and the input terminals of the RCBO.

This is currently up for discussion at the Electrical Safety Council so my advice would be to ask the manufacturer of the DB.

3) If your TT system is derived from a TN-C-S system a single pole RCBO will not disconnect the supply neutral in the event of a neutral to earth fault (it will only switch off the line). This may result in your TT installation being connected to a TN-C-S neutral via the fault - not good if its an out door hot tub http://www.------.org/forums/forum/i/expressions/face-icon-small-happy.gif.

All in all I would fit an S type front end RCD but its your call.


Regards

 

[davesparks]

Schneider specifically state that their SP rcbos are not suitable for TT, but they do also make a decent DP rcbo

 

[Deleted member 9648]

Schneider specifically state that their SP rcbos are not suitable for TT, but they do also make a decent DP rcbo

But why?...given that isolation can be achieved by the main switch and disconnection in the event of a fault will be no different to any other system not employing DP ADS.

By the way...this has come up now due to a problem with unresolved random tripping on a duel RCD board resulting in several loss of freezer contents. The supply is direct from a DNO head and not derived from another supply. Issue as far as freezers concerned has been resolved (for now) by reconfiguring the existing board to include a SP RCBO dedicated to the freezers.

 

[GEOFF S]

Hi

Im with jackhammer, in TT systems, under certain fault conditions you can get a large N - E potential, so double pole isolation on TT is required at some point in system.

Cheers

 

[HappyHippyDad]

But why?...given that isolation can be achieved by the main switch and disconnection in the event of a fault will be no different to any other system not employing DP ADS.

By the way...this has come up now due to a problem with unresolved random tripping on a duel RCD board resulting in several loss of freezer contents. The supply is direct from a DNO head and not derived from another supply. Issue as far as freezers concerned has been resolved (for now) by reconfiguring the existing board to include a SP RCBO dedicated to the freezers.

That would be interesting to know, however manufacturers instructions do not necessarily have intelligent answers behind them!

I can only assume the answer is due to the possibility of higher voltages being present than on a TN system between the suppliers neutral and the earth rod?? But like you said, if this is the answer, what sort of voltages are we talking about, >50V?

 

[Deleted member 9648]

Assuming a TT with a stable low Ra. It would be possible to rely on fuses or mcb's for earth fault protection. Or would it? Why don't SP overcurrent devices have manufacturers instructions that they are not suitable for TT systems as apparently do some SP RCBO's?

 

[davesparks]

That would be interesting to know, however manufacturers instructions do not necessarily have intelligent answers behind them!

That's true, but considering this is from what is effectively the merlin gerin part of Schneider I think it's going to be pretty trustworthy info

 

[HappyHippyDad]

I can't find any specific answer to this!

Here's another suggestion though, funnily enough in response to a question from yourself

See​

post #26
​

http://www.electriciansforums.co.uk...es-electrical-regulations/64662-rcbo-s-2.html

 

[GEOFF S]

Assuming a TT with a stable low Ra. It would be possible to rely on fuses or mcb's for earth fault protection. Or would it? Why don't SP overcurrent devices have manufacturers instructions that they are not suitable for TT systems as apparently do some SP RCBO's?

Hi

Well they are okay, you just also need to have double pole isolation.

Cheers

 

[GEOFF S]

I can't find any specific answer to this!

Here's another suggestion though, funnily enough in response to a question from yourself

See​

post #26
​

http://www.electriciansforums.co.uk...es-electrical-regulations/64662-rcbo-s-2.html

Hi

The answer has been given. On a TT system the Neutral isn't reliably earthed. So if we have a fault, phase to earth, could be a network fault or a property with no RCD, some of the voltage will be dropped accross the Transformer earth Rb, this raises the neutral Voltage. Any other system connected to that neutral will also see the Voltage, which can be considerable. So in a TT you need to isolate both poles.

Cheers