UK Standardising on site to TP & N Sockets rather than TP and no N

[John-]

Hi

I have a site where they have some sockets fed via Three Phase and Neutral and others TP with no N. Rather than both types of sockets for both types of appliances, was looking at 'standardising' and upgrading all plugs to TP and N. And where the appliance did not need a N, it would be left unterminated. That way, the TP no N could still plug into the a TPN socket but only use the TP parts. Hope I am making sense .

My question is, these are all fed by RCBO devices. My concern was around nuisance tripping where N is not being used. I am pretty sure that all phases are monitored so if when an appliance is being used and no N current flowing, then current is still deemed to be balanced as it monitors the current across the phases as well, so will not trip. Is that correct - I am pretty sure it is.

Thanks.

John

 

[James]

There is no problem doing what you intend to do, rcd will work as intended.

However, each socket MUST have a N connected.
It could lead to the destruction of appliances if they require a N and are connected to a socket that is missing one.

Edit, to simplify,
it is fine to use a 5 pin plug and only use the pins you require
It is a danger to use a 5 pin socket with some pins disconnected

 

[pc1966]

My question is, these are all fed by RCBO devices. My concern was around nuisance tripping where N is not being used. I am pretty sure that all phases are monitored so if when an appliance is being used and no N current flowing, then current is still deemed to be balanced as it monitors the current across the phases as well, so will not trip. Is that correct - I am pretty sure it is.

So long as the RCD/RCBO has all 4 wires (3P+N) routed through it then it should work no matter if the load needs N or not.

 

[John-]

Thank you for the responses. Just to confirm. Some cables feeding the sockets will be be TPN others will just be TP. So in some cases there will be no neutral at the socket. So if, someone plugs in to a TP only socket but needs TPN my ecpectation is the device will not function. Or aspects of it (age dependent) will not work. I do not expect dammage to result.

 

[westward10]

It will either not function or it will be seriously damaged.

 

[SJD]

You can't have a TPN socket with no N in the wiring (I think this is what you mean).

Even if all the existing equipment is OK, there is nothing to say someone won't come along with a new item one day with a TPN plug on it, expecting a N and it being seriously damaged, whereupon whoever installed that socket could be liable for the costs.

 

[pc1966]

If you fit a TP+N socket you absolutely must have the N wired up.

As said above in several places, without N connected any unbalanced star load (which one can assume is why it needs N in the first place!) will see widely varying voltages from the open N point to the phases, potentially causing serious damage.

 

[John-]

Thanks for the feedback, and saving the customer the damage. Can someone kindly explain the physics / maths behind this please, as i would like to understand in better detail why the damage will occur. Thanks.

 

[pc1966]

If you imagine the worst-case star with the loads for L1 and L2 being small, say 0.01A each, and the load for L3 being large, say 16A, then both L1 & L2 get ~400V instead of 230V and L3 gets < 1V
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With N present the star point is held at 0V (ignoring Rn for now) insted of being pulled close to L3 phase volts, and to do so it has to carry the ~16A current imbalance.

 

[John-]

Thanks for the detail, much appreciated. So this means that anyone running TPN systems are at risk if the suplier losses a neutral at the mains incomer?

 

[James]

Yes, rural installations are particularly prone to this,
Imagine 9 houses down a lane.
All fed from the same 4 cables along a set of poles
Normally each house would have a phase and neutral from the overhead supply.

Assuming roughly balanced phase loads, the N current will be negligible and it’s voltage will be 0v relative to earth.
Giving you 230v from any phase to N, anywhere along the line.

If you loose the N connection at the transformer end of the cable, the N distributed to all the properties is then able to float to the voltage of the phase with the highest load, each property will have a phase to N voltage being supplied to them that is constantly varying between zero and 400v

 

[John-]

Yes, rural installations are particularly prone to this,
Imagine 9 houses down a lane.
All fed from the same 4 cables along a set of poles
Normally each house would have a phase and neutral from the overhead supply.

Assuming roughly balanced phase loads, the N current will be negligible and it’s voltage will be 0v relative to earth.
Giving you 230v from any phase to N, anywhere along the line.

If you loose the N connection at the transformer end of the cable, the N distributed to all the properties is then able to float to the voltage of the phase with the highest load, each property will have a phase to N voltage being supplied to them that is constantly varying between zero and 400v

Thanks. That makes a lot of sense. The loads in the houses are now the interface (In-between) the different phases and subject to incremental voltages based in the phase difference and load - i think... ?. A bit like putting two phases togethor with a big resistor inbetween.