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Unusual Earthing System
AJJewsbury
1766 Posts
Question
One of the Cahier Technique papers talked about an earthing system, which it described variously as cross between TT and TN-S or an 'impedance earthed TN-S' system, although I don't think either description is particularly accurate. Basically it suggests earthing the supply star point via an impedance, but then Earthing the consumer systems to the same electrode as the source:

66c3de7fa1842c6eefc0499bb3bad3a7-huge-no

As earth fault currents would be low, you'd need RCDs for ADS (just like a TT system), but the way the earthing is connected would seem to give a number of advantages over normal TT or TN systems. The most obvious perhaps is that exposed-conductive-parts remain at (or very close to) true Earth potential even during a L-PE fault - substantially reducing the risks of shock (even for the 5% of the population who aren't necessarily safe under normal ADS) - unlike a TN system where the point of the fault is likely to be at around half the line voltage, and in a TT system anything up to the full line voltage. It also less of an immediate issue if an RCD is a bit sticky or even fails to open altogether. They also claim less risk of damage or fire from L-PE faults than on TN systems - as the fault current is so much reduced.

So firstly what should this system be called? The impedance at the source is like an IT system - but the "T" would mean the consumer has their own earth electrode independent of the source's - which very specifically isn't the case here. Neither is the consumer's PE conductors directly connected to the system neutral which an "N" would usually indicate. As far as I know there isn't a letter to say the consumer's earth is connected directly to the supply's means of earthing (and not N) - maybe we could use "E" or "PE" for that - making it a IPE or IE system? I'm sure there are some better ideas out there.

My other thought was how would such an arrangement fair from the point of view of things like EMI or the effectiveness of SPDs - would it make any difference?

   - Andy.
12 Replies
mapj1
2355 Posts
NER (neutral earthing resistors) are not popular in the UK on the LV side , as we like our supply neutral to be at or near earth potential.Especially with TNC-s🙄
With an NER installation you must treat the distribution neutral as if it may shoot up to single phase voltage relative to true earth, and also all phases as if any two may rise similarly to the phase-phase voltage.  During a fault in which one live of the 3 phases is earthed, this is what happens to the other wires. In effect the phase triangle of voltages stays the same size, or only flexes a bit, but you are now grounding the corner, rather than the star centre.

In practice, this is not normally an issue, except that EMC filters and any spark gaps etc connected L-PE and N-PE need to be rated accordingly.

It also provides a way of riding though certain types of single line fault with less damage occurring. Obviously a 2 or 3 phase fault is as bad as it ever was.
 
Alan Capon
382 Posts
In the British Isles, it is specifically banned by legislation for the LV systems in the Public Electricity Supply. It is widely used at 11kV and often at 33kV to reduce earth fault current, and therefore the cost of cables. At the higher distribution voltages, particularly on overhead networks, there is also impedance earthing using a Peterssen Coil. Here, the system can be stable while one phase is more or less earthed without excessive earth current. The issue is detection of a downed conductor, as it is important to know what the earth fault is. For this reason, the duration of the earth fault will be limited by the protection systems. 

Regards,

Alan. 
perspicacious
364 Posts
OK, having Kirchoff's this idea on paper with some current flow in a consumer side fault to earth, I think I've convinced myself that an RCD would work, but I've still got a nagging thought I may have missed something. Any thoughts?

Regards

BOD
Zoomup
1762 Posts
It can have its advantages......

https://www.youtube.com/watch?v=_JWfM9qB6Uc

Z.
Alan Capon
382 Posts
An RCD would work, as long as the resistor doesn’t have too high a value. In 11kV systems, a resistor of 6.35 ohms is often chosen, limiting the earth fault current to 1000A. 

Regards,

Alan. 
Farmboy
195 Posts

Zoomup:
It can have its advantages......

https://www.youtube.com/watch?v=_JWfM9qB6Uc

Z.

Is there sound to this video, I'm only getting subtitles?

F
Zoomup
1762 Posts

Farmboy:

Zoomup:
It can have its advantages......

https://www.youtube.com/watch?v=_JWfM9qB6Uc

Z.

Is there sound to this video, I'm only getting subtitles?

F

 

Yes Farmboy, there is sound on this video.

Z.
AJJewsbury
1766 Posts

OK, having Kirchoff's this idea on paper with some current flow in a consumer side fault to earth, I think I've convinced myself that an RCD would work, but I've still got a nagging thought I may have missed something.

The loop impedance would probably be similar to that in a TT system - so the RCD should operate in the same manner. I'm not sure if some RCBOs with a FE connection might get upset if they see N drifting significantly away from PE, but that problem is probably manageable even if it does happen.
 

During a fault in which one live of the 3 phases is earthed, this is what happens to the other wires. In effect the phase triangle of voltages stays the same size, or only flexes a bit, but you are now grounding the corner, rather than the star centre.

Agreed. Could that effect be the cause of the problem mentioned in Z.'s video of motors being killed by an open circuit resistor? Would say 400V rather than 230V between conductor and frame be enough to over-strain insulation to breaking point? Or it it more likely to be due to something like capacitive coupling from the HV side of the transformer?

    - Andy.
Zoomup
1762 Posts
lyledunn
372 Posts
It’s an IT system captain but not as we know it!
OMS
694 Posts
It's not a wide spread system type, but I do know of several manufacturing plants that use impedance earthed systems for the plant production power systems - these have combinations of operating as IT or TN systems depending on requirements - eg they may have an earthing resistor imposed to limit fault current and reduce cable screen or earthing conductor sizes to manageable sizes and/or operate as true IT systems with insulation monitoring and no disconnection on "first fault" conditions. I can think of several that operate as 680V systems (particularly where the owner has manufacturing capability in a number of different countries and can standardise on equipment)

From Alan's post above, it should be apparent that selection of 6.35 Ohms as a resistor value reflects the phase to earth voltage of an 11kV System with a desired 1000A earth fault current limit - that resistor may be as simple as a water tank with a defined concentration of bleach added (so called LER's)

Regards

OMS
I came across this system at a large plant in Algeria in the 1970s. It would have been based on French standards from the 1960s. I don't know if it still used either in Algeria or France.
The obvious advantage is that it will survive one PE fault and continue to supply power but there should be an alarm fitted to indicate any large N E voltage which allows faults to be investigated and cleared quickly.
The downside to this is that faults can only be located by turning off the circuits one by one which rather negates the benefit of not having the fault trip the local breaker immediately as in a UK system.

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