Bonding of Glands - star washer vs earth tag

[KenAlmon]

Our offshore gas platform uses a large number of brass cable glands. The cables are braided armour. The glands are designed to bond the armour. What is the experience of others with respect to bonding these glands to ground? To start the discussion, there are a couple of options being looked at:

1. Use an earthtag (or "banjo" - see

http://www.ehawke.com/etag.htm)

to insert over the gland and then run a wire from the earthtag to a grounding stud and from there to platform reference ground.
2. Use a serrated washer and locknut to "cut" into the metallic box and then make sure the box is bonded to the platform reference ground.

The former method is the one that is "recommended" by the vendors but as can be imagined, adds a number of manhours to each box installation. The second method may not have the same guarantee of bonding that the former uses, although both rely on the tightness of the locknut to ensure low resistance in the bonding path.

This discussion is restricted to metallic enclosures only with clearance holes (as opposed to tapered thread hubs).

Comments? Experiences?

 

[desertfox]

Hi KenAlom

It is many years since I worked in switchgear but having drawn many earthing systems for cable boxes, switchgear cubicles etc, there is one thing I remeber reading, possibly in an old ESI standard and that was not to rely on fasteners to carry earth fault currents, the earthing always
had to be carried by the mating materials and not the fixing holding them together. You rightly point out about the lack of guarentee of bonding with the star washer method, can you be absolutely sure that you have broken through any paint work or coating that would be classed as an insulator which you would need to do to form a correct earth path. Also there may be some electrical protection which will operate when there is an earth fault, without a guaranteed earthing run this may fail in its duty.
My advice would be use method 1.


regards desertfox

 

[dbaird]

T & B makes Two-screw ground connection for
armored ground wire.

http://www.tnb.com/

Appleton Electric makes both grounding locknuts and grounding bushings.

http://www.appletonelec.com/

I would think a combination of these devices could be used to accomplish what you want.


David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[sparksski]

Hi Ken Almon

When I used to install armoured cable using glands and tags or conduit etc and washers I would always scrape away the coating on any enclosure back to bare metal to give as good a contact as I could. This seemed to be the normal practice.

Without going back to bare metal you cannot be sure you have a low resistance connection.

Regards

Sparksski

 

[GerH]

You may find that the serrated washer is a dissimilar metal to the brass gland and (possibly) steel box. As water can access the crevice you would have significant corrosion in a short time.

I would focus first on getting a good weathertight seal by having a soft seal e.g. nylon washer between the gland and the box. This would of course mean no electrical bond at the outside gland/box contact.

I would make the earth bond on the inside using either a "bango" tag, bending the tab upwards at 90 degrees so you can easily attach the earth bond wire or using a serrated washer between the gland locknut and the box. I think you can get a plain metal ( usually plated steel ) drilled plate that sits over the gland holes so that good metal/metal contact is guaranteed with serrated washers i.e. excluding paint. Often the inside of gland plates can be plain metal or they can be removed and stripped before fixing any gland.

 

[KenAlmon]

Thanks for your replies!

Ken