Earthing of Three phase Motor

[Buzz1980]

Hi recently was on a job carrying out EX inspections on three phase motor at a pharmaceutical plant. The supply cable going to the motors does not have a core for the earth they have used the braid/SWA of the cable for earthing. So entering the three phase motor with EXe cable glands terminating the earth Via the gland into a threaded entry into 3 phase motor. Now my question/debate I was having with work colleague was yeah it is entering a threaded entry into enclosure where you will get your earth but the motor terminal box is bolted to the frame of the motor through the 4 fixings bolts, is this adequate to carry the earth through to the motor frame or should it really have banjos then separate earth down to the frame. There is a supplementary earth going from the frame to a local earth boss ? Thanks in advance

 

[westward10]

Isn't that the point of the earthing boss?

 

[MJPD29]

Is it braided sy or swa? If it's braid then it is not suitable for earthing

 

[Buzz1980]

The earth cable from the earth boss is just going from the structure of the building to the frame of the three phase motor, it’s not directly connected to the earth braiding of the cable just through all the steel structure of the building ? Thats why I’m thinking they should be linked together ?Also it’s braid that has been used, it is suitable for the installation as this is used in many hazardous area locations for more flexibility

 

[Pete999]

Is it braided sy or swa? If it's braid then it is not suitable for earthing

Feeding a rotating machine with SWA would not be good practices (Vibration) problems, should be a flexible cable of some sort in my opinion.

 

[Deleted member 26818]

Sounds like they’ve earthed the motor via the separate earth, which sounds more like bonding than earthing.
What’s the measured Zs?

 

[Buzz1980]

Yeah that’s what my view is they have earthed the frame with supplementary bond, but the motor terminal box is earthed through the glands and metal to metal flange of the terminal box and the frame are bolted to each other given continuity through to frame from the terminal box. Would this be adequate ?

 

[westward10]

Assuming this is an Ex rated motor I would consult the manufacturer.

 

[Deleted member 26818]

Yeah that’s what my view is they have earthed the frame with supplementary bond, but the motor terminal box is earthed through the glands and metal to metal flange of the terminal box and the frame are bolted to each other given continuity through to frame from the terminal box. Would this be adequate ?

If it’s braid, it’s not reliably earthed.

 

[MJPD29]

Feeding a rotating machine with SWA would not be good practices (Vibration) problems, should be a flexible cable of some sort in my opinion.

I agree but if flexible sy it will need an earth core or separate earth

 

[Bobster]

SY isn't compliant in an Ex environment for LV

 

[Buzz1980]

It’s not SY it’s ships wire braid it’s used all the time offshore and in most hazardous area locations. Don’t know the exact Zs as can’t do any live testing and that will be carried out by commissioning, we are just out to do EX inspections but was needing to know regarding the earthing arrangement ?

 

[Andrew Hunter]

I would if installing this use 4 core ie seperate core for earth. if as you say just inspecting then the minimum i would expect is a seperat earth from supply into motor through seperate gland (Atex stuffer) Not Equipotential bonding.

 

[7upsgreat]

SY isn't compliant in an Ex environment for LV

I've seen loads of SY cable used in Ex environments serving motors. Out of interest why isn't compliant and where's this dictated?

 

[Darkwood]

I've seen loads of SY cable used in Ex environments serving motors. Out of interest why isn't compliant and where's this dictated?

The braiding of your bog standard S/Y you could pick up at your local wholesalers is a screen not an earth, it is no to be used as an earth but should be earthed itself (low voltage applications), S/Y is a type of cable design and strictly speaking saying it cannot be used as an earth is incorrect, it comes down to how it has been design and manufactured, there are S/Y's that are suitable for the braiding to act as the earth, however, these are not the S/Y's we can pick up in any wholesaler.

S/Y, C,Y and Y/Y are how a cable construct is arranged and many manufacturers out there have their own specific versions with their own individual design and environmental tolerances.