HV/LV distribution earthing 1

[bspace123]

Hi all,

I'm trying to understand earthing requirements in a high/low voltage (6.6kV/0.4kV) installation. Can someone please clarify/confirm my understanding is correct based on some research I've done:

- For all high voltage equipment "conductive parts" (e.g. motor frames or terminal boxes) supplied with copper screened PVC insulated power cables, the copper screen is connected to the motor earthing terminal and acts as the protective earthing conductor for the motor. Obviously it is bonded to the earth system at the motor supply point. Is there an additional requirement to have a separate earthing conductor connected from the supply origin to the exposed parts?

- For all "extraneous parts" subject to EPR (e.g. metallic structures with underground concrete footings) they must be bonded to Earth. The earth conductor must be sized for the maximum available earth fault current, which may cause the EPR. Is there a requirement to bond these structures at two independent points ? (e.g. two individual bonding conductors connect the structure back to the earth grid)?

- For all extraneous parts not subject to EPR (e.g. metallic doors, removable metallic floor grating etc.), they must be bonded to Earth. I've been reading that this earthing conductor must be sized to half the size of the of the supply earth that the parts may be subjected to under fault conditions. For example, if temporary cables will be run through a door to supply test equipment, then the bonding conductor must be sized to at least half the size of the earth cable in those temporary cables (or 6mm2 for mechanical integrity), whichever is greater. Is there a requirement to bond these structures at two independent points?

Are there standards which specifically reference these scenarios and what the correct practice is?

 

[davidbeach]

It will depend, at least in part, where you are and which codes and standards apply. With those voltages I’m guessing that an answer from the NEC won’t help.

 

[bspace123]

This is in Australia, so Australian standards.. but our standards are mostly based on IEC if no one is familiar with Australian standards.

 

[waross]

The primary purpose of the copper screen or a grounding conductor included with the power conductors is to draw enough fault current to trip the supply breaker.
However when a fault occurs at a motor the power conductor and the earthing conductor form a voltage divider.
Many of our power cables have an included grounding conductor that is two AWG sizes smaller than the power conductors.
As one leg of a voltage divider the voltage on the motor frame may rise to over half of the applied voltage.
In the case of a 480/277 Volt circuit the applied voltage is 277 Volts and a voltage to ground of 277/2 = 138.5 Volts or higher may appear on the motor frame until the supply protection trips offline.
This is the reason that some codes, regulations and engineering standards require a separate grounding conductor often with a greater ampacity than the minimum allowable by the NEC or CEC.
I hope this helps in your understanding.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bspace123]

Thanks for the response. But the voltage rise of the motor frame would depend on the ohmic value of the earth conductor. Adding an additional earth would only reduce this value, hence reduce the amount of potential rise of the frame. Having a read a few more regs, the only requirements I'm aware of from any regulation is to ensure this ohmic value is below a specific limit (for exposed and extraneous earthing) and that the ohmic value of the earth loop during a fault is low enough to trip the upstream supply breaker within a specific time frame (for exposed part earthing). Earth conductors obviously also need to be rated for the available fault level.

My only questions now are:

- can cable copper tape screens be used as the protective earthing conductor for exposed parts of high voltage equipment on the provision that the above criteria are met? Or is there a requirement to have a separate earthing conductor?

- do extraneous parts both subject to epr and not subject to epr need only be earthed at one point provided the above criteria is met? Or is there a requirement to earth at two independent locations?

 

[7anoter4]

There are some different situations in which a medium voltage cable is supplying a motor.
In any case, the cable shield has to be grounded at-at least-in one point.In this case the open end has to be insulated in order to avoid potential transfer-even if the load current build-up voltage was treated. If it is grounded at both ends a circulating current will flow through the shield -even in steady load state-and it may reduce the cable ampacity then you have to oversize the conductors.
In short-circuit case a part of this current returns to the source through the shield, then reducing the EPR at supplied installation location.
In any case the shield it is not an equipment grounding conductor-not for earthing and not for bonding. You have to use earthing and bonding conductors as per EN 50522 and ENA Engineering Recommendation EREC S34.
The touch and step potentials have to be checked. I'd prefere to do it according to IEEE 80/2013 Annex D

 

[ScottyUK]

bspace123,

Under UK ways of doing things I would expect that a separate fault-rated tape or bare copper conductor would run alongside the supply cables. Any contribution from the shield would be incidental and not part of the designed fault path. Local bonding to earth and structure is usually at diagonal corners of the equipment in a minimum of two locations. If you can find BS 7430 it's quite useful - there may well be an Aussie equivalent.

The IEEE 80 calculations are fairly straightforward, but they don't definitively make you code-compliant against the BS / AS / IEC codes. They're simple enough though, and it gives you a reasonable idea of you an earthing grid or array will behave. For a proper answer you'll need one of the software modelling tools like CDEGS, or to outsource it to someone who does that type of work.

 

[7anoter4]

Thank you Scotty for your remarks.I think you meant BS 7430/2011 clause 9.12 Earthing of sheath and/or armour of cables.
Also IEEE 575/1988 5.4.3 Parallel Ground Continuity Conductor[I hope the new-2014-edition did not delete this chapter.] it recommends such a parallel grounding conductor.
As I understood- this conductor is required only for single-core cables.
By the way, BS 7430 recommends BS EN 50522 on section 5 HV/LV interface and Annex A Guidance on typical HV/LV interfaces.
Since BS 7430 does not indicate how to calculate the EPR within a grid or external ‘touch’ potential at the fence, ENA EREC S34 recommends how to do it.
[ENA=ENERGY NETWORKS ASSOCIATION].