Type A RCBO?

[RyanM]

Thanks for the replies.

The charger will be outside, so does need a double pole RCD. I can't see anything like that on the schneider site to directly fit in to the board.

In terms of availability, will it be best to install a MCB in the DB, then in an adjacent enclosure install a double pole RCD, then an isolator feeding the charger?



Thanks a lot[/list]

 

[Spark123]

It is beginning to look that way, Schneider make a DP RCBO but looks like it is only available in class AC.

 

[ericmark]

It is beginning to look that way, Schneider make a DP RCBO but looks like it is only available in class AC.

They do make a DP RCBO type type SI as I have said but once you realise it needs it's own enclosure then the simple MCB and separate box with RCD would seem simple way around the problem.

The next question is the board of the type where you can take a non RCD protected supply from it. I would assume it is since you were going to use a RCBO.

To me the whole idea of a TT supply from a TN-C-S is fought with problems. One has to consider what can be touched simultaneously from both earth systems and the voltage gradient under fault conditions. The theory is great because of fire risk caravans and cars are not parked right next to premises so there is no problem. However in practice cars and caravans are parked right up to the house and one has to look for any down spouts or similar which may be connected to the house earth.

So I would do a risk assessment and not blindly follow the recommended TT earth system but decide which is the most appropriate in your case. With the charger across the road from me the electric, gas, and garage door all metal and earthed are within 2 foot of where the van is parked so personally I would not use TT.

 

[JohnW2]

To me the whole idea of a TT supply from a TN-C-S is fought with problems. One has to consider what can be touched simultaneously from both earth systems and the voltage gradient under fault conditions. The theory is great because of fire risk caravans and cars are not parked right next to premises so there is no problem. However in practice cars and caravans are parked right up to the house and one has to look for any down spouts or similar which may be connected to the house earth. ... So I would do a risk assessment and not blindly follow the recommended TT earth system but decide which is the most appropriate in your case. With the charger across the road from me the electric, gas, and garage door all metal and earthed are within 2 foot of where the van is parked so personally I would not use TT.

... but what about the alternative - is that necessarily much different or any 'better'? There are countless things outdoors which are unavoidably in continuity with 'true earth' and which, in most cases, are not 'bondable' - the ground itself, metal fences/gates, even a metal spade or fork sticking into the ground. If one exports a TN-C-S earth to the charger/car (because one is not happy to TT it), is there not going to be the same sort of of scope for there to be potential differences between 'two earths' that could be touched simultaneusly, just as there would if of TTd the charger/car ?

Kind Regards, John

 

[chrisdicko]

This is quite an interesting thread, as it's something we are looking at doing.

If you were to mount a charger approx. 4m away from the building, mounted on a metal post, which is sunk in to the floor, would you give the metal post it's own earth direct from the DB, or would the earth from the SWA be sufficient?

 

[ericmark]

If the charger post and car are both 4 meters away from house then likely would use TT supply even if right against the house with no earthed extraneous conductive part within reach still it would be TT where the question starts to get cloudy is where there are earthed extraneous conductive parts within reach of the car or charging post. The real answer would be to convert the whole house to TT as long as there are no earthed extraneous conductive parts which can be touched on the one house and next house along in which case the whole street would need making TT.

It is so easy for items which were not earthed extraneous conductive part to latter become a earthed extraneous conductive part for example when my father-in-law had his garage door converter to electric opening. The answer would be to change door to non metallic but there is clearly no one answer fits all and the only way is to do a risk assessment of the individual installation and use ones skill and training to decide which is the most appropriate method.

There is of course the other option just to say sorry can't be fitted there! With the house across the road he did have a garage and the supply could have been installed inside the garage. However that raises another question "How safe are the car batteries is there a chance they will bust into flames like Dell computers did or Boeing aircraft did?".

So clearly all down to a risk assessment its a big can of worms and as "JohnW2" rightly points out there is no definitive answer.

 

[Spark123]

It is beginning to look that way, Schneider make a DP RCBO but looks like it is only available in class AC.

They do make a DP RCBO type type SI as I have said but once you realise it needs it's own enclosure then the simple MCB and separate box with RCD would seem simple way around the problem.

The one I was thinking of goes directly into a Merlin Gerin type A board taking up two module widths, unfortunately I can't find a pic on the Schneider website to ensure the neutral positions are the same, here is a scan of the old Merlin Gerin catalogue page:

Untitled

• Spark123
• 17 Sep 2013
• 1

I think it has just dawned on me - the photo in the catalogue is the wrong one! It shows a Single Poled version - good old Merlin Gerin!

 

[JohnW2]

The one I was thinking of goes directly into a Merlin Gerin type A board taking up two module widths, unfortunately I can't find a pic on the Schneider website to ensure the neutral positions are the same, here is a scan of the old Merlin Gerin catalogue page:

I don't see anything on that page which says that it is a Type A (or Type SI, if that's the same) device - am I missing something?

Kind Regards, John

 

[JohnW2]

If the charger post and car are both 4 meters away from house then likely would use TT supply even if right against the house with no earthed extraneous conductive part within reach still it would be TT where the question starts to get cloudy is where there are earthed extraneous conductive parts within reach of the car or charging post. ... So clearly all down to a risk assessment its a big can of worms and as "JohnW2" rightly points out there is no definitive answer.

Indeed, but I suspect that, in practice, TTing the charger/car would very commonly 'win' over exporting a TN-C-S earth. Particularly if there are no metal downpipes or an electric metal garage door (or a bonded non-electric metal one), there are rarely going to be any things outdoors which will be connected to a TN-C-S earth which is touchable at the same time as an outdoor TT earth. On the othe hand, as I said, routes to true earth are essentially unavoidable outdoors, so there is always going to be a risk of someone being in contact with those simultaneously with touching a TN-C-S earth which has been exported to charger/car.

Mind you, this is all rather theoretical. Whichever route one chooses, the chances of there being a dangerous potential difference between a TN-C-S 'earth' and 'true earth' at the very moment that someone touches both of them must surely be 'vanishingly small'?

Kind Regards, John

 

[stillp]

Talking to charger suppliers at the Low Carbon Vehicles show at Millbrook a couple of weeks ago, they all said they TT every installation.

 

[Ahtyrrell]

What about adding earth rods to the TN-C-S system effectively creating a PME then everything can be on one system and should be at the same potential,
Its also worth nothing that every electric garage door actuator I have see has been class II not saying they all are but I would say most are, so the actual door doesn't need to be earthed
All of this really highlights what a poor idea TNCS/PME earth really is as it doesn't really provide a safe earth system at all

 

[JohnW2]

Talking to charger suppliers at the Low Carbon Vehicles show at Millbrook a couple of weeks ago, they all said they TT every installation.

That doesn't surprise me. As I just wrote, I would expect that the risk assessment would more-often-than-not come down on the side of TT (as opposed to exported TN-C-S). Whether theres much to choose between TT and TN-S is less obvious. However, as I just wrote to eric, one might be more likely to win the Lottery than suffer significantly as a result of exporting a TN-C-S earth!

Kind Regards, John

 

[stillp]

I should add that one of the charger suppliers at the LCV show mentioned 2 problems with TT-ing; one being contractors who drive earth rods through underground services, the other being contractors who simply lay the earth rod in a service duct, to avoid damaging underground services!

 

[JohnW2]

What about adding earth rods to the TN-C-S system effectively creating a PME then everything can be on one system and should be at the same potential,

That would help a bit. However, if (very rarely) the TN-C-S earth rose to a very high potential relative to true earth, that wouldn't change a lot as a result of a few earth rods - so one would probably have to put in rods 'all over the place' the ensure that there were no remaning things at/near truth earth potential that could be touched simultaneously with the exported earth - i.e. to attempt to raise the potential of the entire garden (or whatever) to TN-C-S 'earth' potential..

Far simpler, IMO,to just TT the charger.

Kind Regards, John

 

[Adam_151]

question "How safe are the car batteries is there a chance they will bust into flames like Dell computers did or Boeing aircraft did?".

I did a job for an insurance company once (or rather to the main contractor that was working for the insurance company) of putting back together a posh double garage (plastered walls, tiled floors, separate workshop bit painted harley colours - guy owned a car dealership) after he'd left a car battery on charge for weeks on end and it had eventually caught on fire, fire mostly contained to garage, but smoke damage to the next room