Please help me understand my new house electrics

[LiamPope]

I found this interesting reading on the subject...

http://www2.schneider-electric.com/...ility-safety/low-voltage-minus-1kv/ect173.pdf

It even mentions Norway being IT a few times. Still doesn't mean I have it I guess.

What I can't quite get my head around is how detection of a first fault can be managed where it is a domestic network and the consumers are private houses. Seems like RCDs should operate on a first fault as there should be some current flowing through network stray capacitance - the supply is not truly isolated. But most places I've seen don't have them! I've got one but it is only for the basement.

Oooh there are some steel boxes outside the property with 'lightning' symbols. One per property. Not sure what they are.

This is all interesting, and I'm curious about all these details, but also just want to know if my installation is safe. I guess the Norwegian equivalent of a PIR is what I ought to do. I just hope I get an electrician who a) is well clued up on the network side and theory in general, and b) who speaks English. Probably a similar chance of finding that over here as back in the UK

 

[JohnW2]

What I can't quite get my head around is how detection of a first fault can be managed where it is a domestic network and the consumers are private houses. Seems like RCDs should operate on a first fault as there should be some current flowing through network stray capacitance - the supply is not truly isolated.

Do you mean if the house's 'earthing system' was connected to earth (e.g. a rod) and that fault current would then flow through stray capacitance between earth and the network cables? If so, that's obviously conceptually possible, but at 230V and 50Hz, to get 30mA to flow would, by my quick calculation, require a capacitance of around 0.4μF - which I would have thought was an awful lot for 'stray capacitance.

However, as has been suggested, even if it is 'IT' (which it strictly only would only be if you had a local earth rod, to provide the 'T'!), there might be a 'high impedance connection' between the supply transformer's neutral and earth, specifically put there to enable RCDs to operate in response to 'first faults' in supplied installations.

I would think, as you say, that a good starting point would be to talk to someone who actually understands Norwegian supplies!

Kind Regards, John

 

[mfarrow]

True, if it's a 3-phase supply, then there might not be a neutral.

There wouldn't be one on a 1-phase (2-phase in reality) IT circuit, would there?

Adding a high impedance path to earth from the neutral of an otherwise floating supply certainly allows an RCD to operate in response to the 'first fault' - but if that's the only fault, I'm not sure that I see the point/need.

Agreed, there isn't much point of IT in a domestic situation, it's all a bit academic.

As for the 'second fault', I'm not sure what you/they have in mind. As I've said, provided the exposed-c-ps are all connected via CPCs (even if the CPCs aren't connected to earth or anything else), if faults from two different live conductors to exposed-c-ps arise, then an OPD will provide ADS, won't it?

Yes it will, but I think we need to go back to the principles of the wiring regulations/legislation etc. You shouldn't expose anyone to a voltage >50/60V (I forget which), and I'm pretty sure the 'rules' don't distinguish between a voltage of >50V coming from an earth-referenced supply or a non earth-referenced supply, unless you can tell me otherwise.

Hence, IT cpcs are connected to earth to prevent this >50V potential occurring. From an engineering perspective it may seem a bit daft I know, but I suggest it may be due to the potential for any installation or situation to introduce an earth reference, from a water/gas pipe to using your old 1950's lawnmower to cut the damp grass.

 

[JohnW2]

There wouldn't be one on a 1-phase (2-phase in reality) IT circuit, would there?

Maybe we're just into semantics, but I don't see why one can't have a 2-wire IT supply.

Agreed, there isn't much point of IT in a domestic situation, it's all a bit academic.

In the UK, I agree, but (in the context of this thread) seemingly more than 'just academic' in Norway.

Yes it will, but I think we need to go back to the principles of the wiring regulations/legislation etc. You shouldn't expose anyone to a voltage >50/60V (I forget which), and I'm pretty sure the 'rules' don't distinguish between a voltage of >50V coming from an earth-referenced supply or a non earth-referenced supply, unless you can tell me otherwise.

For a start, you are presumably talking about UK regulations/legislation. Like you, I know a bit about that, but nothing about Norway. However, in terms of what you say above, 'voltage' means nothing, without a reference. It's potential difference that matters. When you say that one shouldn't expose anyone to a 'voltage' >50/60V, you are talking about a potential difference of 50/60V between (presumably) an exposed-c-p and what??

Hence, IT cpcs are connected to earth to prevent this >50V potential occurring. From an engineering perspective it may seem a bit daft I know, but I suggest it may be due to the potential for any installation or situation to introduce an earth reference, from a water/gas pipe to using your old 1950's lawnmower to cut the damp grass.

If the supply really were totally floating, then 'introduction of an earth reference' would surely be totally irrelevant, and of no importance? 'True earth' surely has no significance if no part of the supply transformer secondary is connected to earth?

I continue to be very confused about all of this!

Kind Regards, John

 

[ericmark]

On a few odd machines we did have IT as well as the Hong Kong TBM and in the UK it was always part of a control system and in the operators cab there would be an earth leakage warning lamp the unit working the warning lamp was the only earthed or grounded point.

The idea is until the first fault neither line supply will give one a shock only after first fault is there a problem and since continually monitored it is safe enough have the advantage that with a fault it will not trip out the concrete batching plant as if it did it would require manually digging out the motors are not strong enough to re-start the plant when full.

When we got a fault it took us all week end to find it. Intermittent of coarse the main problem was although two lines it was wired same as line neutral with no switches on the second line so one had to painstakingly remove the line 2 connections until found. Turned out to be water in the three pin socket connecting the solenoid for the air horn.

The plant was of German design and I am sure it did not comply with UK regulations but the Germans said it complied with theirs and there is a directive that we should allow European equipment to be used in this country.

IT stands for Isolated Earth (Using French name for Earth) but clearly to monitor it does need an earth connection although high impedance. The simple idea for the DC cranes in the steel works was two bulbs with centre connected to earth while no first fault both bulbs glowed but if there was a fault one would come on bright.

However with both batching plant and crane there had to be an isolation transformer feed just that item. It is of course the same with a bathroom IT supply for a shaver it needs a dedicated isolation transformer.

So to work with a house each house would need it's own transformer you could not share a transformer with a neighbour. I have seen this once in the UK for USA servicemen's houses each house had it's own step down transformer to supply 120 - 0 - 120 volt but that was rather an odd situation.

the residual current monitoring devices like shown here are nothing like the standard RCD and in the main you can only have one RCM per secondary winding of the supply transformer as it will show a second device as being a fault.

As I said before yes we do have IT supplies but other than the shaver socket it is really not something one would use with domestic premises. If you have a standard RCD then can't see it being a IT supply.

It does say where IT is used it is because they can't earth so if you do have an IT supply clearly nothing you can do anyway.

 

[JohnW2]

... there would be an earth leakage warning lamp the unit working the warning lamp was the only earthed or grounded point. ... The idea is until the first fault neither line supply will give one a shock only after first fault is there a problem ....

As you say, with IT there is no problem (shock risk) after the 'first fault'. However, as I've also said, I don't yet really understand what sort of 'second fault' is being contemplated that would result in a persisting (uncleared) shock risk. As I've said, if the first fault were a fault from one live conductor to one exposed part, and the second fault were a fault from a different live conductor to a different (but, directly or indirectly, simultaneously touchable) exposed part, then provided one had CPCs joining the exposed parts, that mean that the second fault would cause an OPD to operate (hence clear the faults), even if the CPCs were not connected to earth.

IT stands for Isolated Earth (Using French name for Earth) but clearly to monitor it does need an earth connection although high impedance.

Not quite. The first letter ('I') means that the supply is isolated from earth. The second letter ('T') means that the installation is locally conncted to earth, independent of the supply. That's why I said that what has been described to us by the OP is not strictly even 'IT' unless his installation is connected to an earth rod.

However with both batching plant and crane there had to be an isolation transformer feed just that item. It is of course the same with a bathroom IT supply for a shaver it needs a dedicated isolation transformer.

No. As above, an isolated supply such as a shaver socket is not 'IT'.

So to work with a house each house would need it's own transformer you could not share a transformer with a neighbour.

Again, an IT system does not require that each installation has its own isolation.

Kind Regards, John

 

[bernardgreen]

From the distant past I vaguely recall "floating supplies" ( not referenced to earth at the substation ) being used to supply un-attended communication equipment buildings. Some of the equipment had connections to the local ground mat. These grounds were not for safety but for the best operation of the equipment.

There was ground fault detection which alerted a control room but how this was achieved I cannot recall.

 

[JohnW2]

From the distant past I vaguely recall "floating supplies" ( not referenced to earth at the substation ) being used to supply un-attended communication equipment buildings. Some of the equipment had connections to the local ground mat. These grounds were not for safety but for the best operation of the equipment.

That makes sense. As you imply, communications equipment often needs a true earth for reasons totally unrelated to safety.

There was ground fault detection which alerted a control room but how this was achieved I cannot recall.

It certainly would not be totally straightforward because, at the least, one would have to detect faults between any live conductors and exposed parts/CPCs - and, ironically, the monitoring equipment would probably have to introduce 'moderate impedance faults' in order to facilitate the monitoring, since hi-Z monitoring equipment would pick up spurious voltages (due to capacitance etc.)!

Kind Regards, John

 

[LiamPope]

OK now i'm more confused . I popped off a cover to investigate further and I measure 225 V between all phases as if it's a delta connected system. All the twin pole MCBs are just wired up between two phases.

So how the heck is my cooker hob 380 V?? Maybe it isn't - need to get another cover off to check, but it's clearly labelled as such on the appliance???

 

[JohnW2]

OK now i'm more confused . I popped off a cover to investigate further and I measure 225 V between all phases as if it's a delta connected system. All the twin pole MCBs are just wired up between two phases.

You mean 225V between each pair of phases? If so, I guess you have a "225V 3-phase, 3-wire" - i.e. if you had a neutral (which it seems you haven't) there would be about 130V between each phase and neutral. It's one way of getting 225V for the final circuits, though!

So how the heck is my cooker hob 380 V?? Maybe it isn't - need to get another cover off to check, but it's clearly labelled as such on the appliance???

I can't see that it can be, with that supply. It could have a 3-wire, 3-phase "225V" supply, or else a 2-wire 225V supply from one pair of phases.

Kind Regards, John.

 

[ericmark]

I seem to remember seeing a USA system where a delta secondary is used but one winding has a centre tap to earth. So two phases where 127 volts to earth but the third phase was much higher seem to remember about 180 volts to earth and was called the hot phase.

Fig 2.6 shows an earthing impedance but does not show how it is connected with the system I worked on in Hong Kong on a Robbins TBM there was no connection to earth on the delta output of the transformer. If a star secondary was used then having a high resistant to connect the star point to ground would work but with delta it would need three resistors one between each phase and ground which seems an odd set-up.

But I still say asking on a UK forum is the wrong place to ask. I looked up Robbins and they are USA based. The batching plant with IT was German but I think it was built for export and did not even comply with German regulations.

I did hate working in IT systems but as to safety it would be hard to say if better or worse than TN-C-S at least no fault could raise the earth wire to 400 volts above true earth.

 

[JohnW2]

But I still say asking on a UK forum is the wrong place to ask.

Indeed so ...

I would think, as you say, that a good starting point would be to talk to someone who actually understands Norwegian supplies!

Kind Regards, John

 

[ban-all-sheds]

the third phase was much higher seem to remember about 180 volts to earth

208v