3.3 Kv cable grounding

[Rizgar]

Hello,
Is there needs to ground medium voltage (3.3Kv) cables of size (3/C-185 sqmm + 3/C-70 sq mm) which connecting switch gear to 10 MVA transformer. The switch gear is feeding 3.3Kv motors through 3.3Kv motor control(circuit breakers and soft starter RVSS). Is it important to ground such cables?why?
Knowing that the transformer, switch gear, motor control and the motor all grounded.
Thanks,

 

[amps21]

If you are talking about the conductor sheath grounding, i think:

Due to varying magnetic fields there are induced voltages in the sheath and other conductors in the vicinity. Thus, The sheath should be grounded for safety reasons. There are a number of ways to ground though. Single, Both terminals and Cross (which involves transposing the sheath at equal intervals). There may be other methods which i am not aware of. But remember for longer runs using single point ground, may cause high voltage may be induced at the free end which should be considered. Grounding both ends shall form a closed circuit and may cause circulating currents which should be considered. I think there have been many posts in this forum which talk about this in detail.

 

[waross]

Hello Rizgar
A couple of points.
All cable sheaths should be grounded at the supply end.
The induced voltages and possible resulting currents that amps21 discusses are a serious issue with single conductor cables.
There is not a problem when all the conductors in a circuit are contained in the same cable.
At the load end, it is a case for judgment. If the cable sheath does not have sufficient ampacity for equipment grounding (and it often doesn't) you may want to leave it floating at the load end and use a separate conductor for equipment grounding.
Leaving the sheath floating on a 3.3 Kv cable can allow any insulation leakage current to energise the sheath of the cable.
yours

 

[Rizgar]

Thanks amps21 and waross for replying.
Waross do you mean that leaving the load end float is better.
Really there is 9 cables of that size which connecting the switchgear to 10MVA transformer.The switchgear connected to 4 feeders that feeding Medium voltage motor control by Cables of same rated voltage (3.3 Kv)and the motor controls are feeding motors of 933 Kw (1250Hp)also by 3.3Kv cables. So shall I connect the sheath of the cables that feeding the motors on the supply end (Motor control center) and leave it float at the load end (Moror).

 

[waross]

I understand that you have multi conductor cables with all the phase conductors contained in each cable.
Your local codes probably state the method of grounding. I am not familiar with IEC codes.
My experience is to ground both ends of multi conductor cables.
Many types of connectors inherently ground the sheath when installed, however, it is possible to install cables without the need for connectors.

In the case of single conductor cables, amps21 has given an accurate description of the factors involved.
yours