Cable Armor Grounding

[ksps100]

Hi All
We are using Steel Wire Armored cable in our project.
Cable Glands are used to fix the Cable to the Switchgear at the Switchgear end and at the Motor hub also.

We use Brass earth tags to connect the Armor to Ground at the Switchgear end.

Is it compulsory to Ground the Armor to Ground at the Motor end also.

Since the Cable gland is connected at the Motor hub directly and the motor body is connected to ground,we feel that it is not required.

Could anybody guide me regarding the applicable specification as per NEC/IEC etc for this matter.
Thank you

 

[ScottyUK]

In the UK I would expect to see the armour grounded at both the source and load end. In the UK it is common for the armour to be used as the earth conductor, although it is good practice to use a copper conductor as the earth and not rely on the armour. On larger cables it is more-or-less mandatory to use an additional copper conductor because the armour does not have adequate cross-section. Your bond earth must return to the source of the supply (e.g. the MCC or switchgear )and not be just a supplementary bond to local steelwork. If you were to treat the cable as a TT supply then you could avoid running the earth conductor but I have not seen this done for motor applications. TT systems are not a preferred installation and bring their own set of problems.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[cflatters]

Based on UK Regs - ie BS 7671 (a subset of IEC364), you should ground the cable armour at both ends, you may need a supplementary earth (if the area of the cable armour is not adequate according to BS 7671 Section 54)

I have been in the contracting and consulting industries for over 25 years and have never dealt with a TT or an IT system for that matter.

Note : BS 7430 also provides useful guidance on earthing and bonding arrangements.

You dont mention voltage - I assume you are talking LV, if not BS7671 does not apply, though the principle is still the same regarding earthing of cable armour, and BS 7430 is still relevant.

 

[ksps100]

Thanx all for your replies:

The base condition is explained again:

All Cables are 3Core + PE
For eg: 3C(185mm2) + PE(50mm2),Copper conductors, XLPE Insulated,PVC Outer Sheath.
The Cable Armor is 0.8mm thick.

The PE Conductor is connected to the Grounding Bar at the Switchgear end and is also connected to the Ground terminal inside the Motor Terminal Box.
I suppose that takes care of the Earth Conductor requirements.

The issue is the Armor ground of the cable
( to prevent any leakage currents from causing any nuisance tripping during energization of Motor feeder cables)
(We have feeder cable lengths of approx 380 Mtrs especially for the Medium voltage 4.16kV motors and 13.8kV Motor)

When the Cable has a PE Conductor then should the Armor still be grounded at Both ends?

Please advise again.
Thank you

 

[fsmyth]

Can't speak for power practices, but in the signal
world, one only grounds at most one end of a cable
shield, at the most effective (i.e., quietest)
point in the system. In practice, generally it
does not matter that both ends are grounded, as
long as there is not a significant potential
difference between the two ends.
And on a supply only cable, in which induced
noise is almost a non-issue, I personally would
feel better about the additional ground conductor.
<als>

 

[Kiribanda]

ksps100,

I think you are following IEC/ IEE wiring practices.

Therfore it sound like your MV/ LV earthing system is TNS. That is why you have laid a PE from source to the load to take the earth fault current back to the source.

But as per IEE 15 th edition you are allowed to use the cable armour (which is basically considered as a mechanical protection)as the earth return conductor in TNS system provided that the armour is PROPERLY SIZED for the maximum earth fault current expected at the motor terminal.
Since you have already laid a seperate PE with 50 sq. mm cable, all what you have to do is to connect the armour at the motor terminal to the earth terminal of the motor. By doing that you will enhance the PE conductor by another wire. Our final target should be to allow ADEQUATE earth fault current to flow to operate earth fault protection device in correct time.

The earth wire connected to motor body is just a "SAFETY EARTH". We should not depend on that during earth fault conditions.

Similarly in the NEC world whetehr we use TC (non armoured) or TECK (interlocked armour) we have to provide a seperate grounding conductor with the phase conductor(s) sized per Table 250.66. But armour is still properly connected to ground at both ends!

But in the LV signal world, some manufacturers donot agree to ground the armour/ shield of signal/ data cables due to noise problems. We cannot apply the same theory for power conductors.

Kirianda