Ground strap required on external cabinet doors? 12

[HighPotential]

Was wondering if anyone knows if there is a code REQUIREMENT for ground-bonding straps ACROSS cabinet door hinges bonding the metallic door to the enclosure as oppossed to simply grounding through the hinges.

Any advice would be appreciated,

Thanks

 

[Wiggie]

According to a major switchboard manufacturer, a ground strap is not a normal requirment by most customers and users, and that only West Coast utilities require a bonding strap from the door to main enclosure.

 

[RajT]

Wiggie is correct in his statement.
Most doors on switchgear are bolted by metal hinges which provides adequate groung continuity so further strapping is not necessary.
Engineers often specify and insist the requirement not knowing that its just a folly

 

[jbartos]

Suggestion: Visit

http://www.ormec.com/motion/mktdocs/shldgrnd.htm

For: Use ground straps made from 1 inch (25 mm) silver tinned flat copper braid to connect the cabinet door(s) to the enclosure, the first sub-panel to the enclosure, and from each sub-panel to the next.

 

[busbar]

Reliable continuity for fault currents and limited potential difference cannot be consistently achieved solely through door hinges—given routine paint, lubricant and corrosion.

 

[kevd]

I totally agree with 'busbar'.

In europe hinges would be classed as a "fortuitous" earth. To ensure touch potentials are low whilst fault currents flow, a positive mechanism for earthing must be applied using a strap. Hinges are not designed to carry current, so cannot be gauranteed. I wouldn't touch it!!!!

For EMC protection, straps are no good, you must use an EMC gasket to totally seal the door, but that's another story/thread..............

 

[jghrist]

busbar and kevd,

What you say may be true, but it is not normal practice in the US to provide a bonding strap to panel doors.

The link given by jbartos is concerned with EMI protection of sensitive equipment and 300 MHz harmonics not safety for faults.

 

[RajT]

As jghrist points, if it is not normal practice then why is it so!Is it that the above experts are correct and the whole of the switchgear industry blind.
The components (usually metering) rarely exceed 220v and backed up by a fuse not exceeding 10A, so where are these high fault currents coming from!
They never have bonding straps on washing machines/dryers, so does that make them dangerous!

 

[HighPotential]

RajT,

I think high fault currents are not the concern. 120V is enough to really hurt someone and is not so high as to be able to "drain" across a corroded hinge. So if someone grabs the door, whammo, "Hello 120V".

Thanks for all the advice!

 

[Wiggie]

In my experience, metal doors are bonded to the enclosure if instrumentation is installed in the door. Otherwise, a bond strap is not needed.

 

[peebee]

Re: "As jghrist points, if it is not normal practice then why is it so! Is it that the above experts are correct and the whole of the switchgear industry blind."

There's more than one way to skin a cat. There's more than one way to ground a door. Some are better than others, but many can be considered sufficient depending on the application. Beauty is in the eye of the beholder.

 

[HighPotential]

What's with all the stars?

 

[busbar]

The “West Coast” mention may be related to EUSERC metering standards for low- and medium-voltage metalclad sections that periodically require energized access by unsuspecting utility folks.

No offense, but low-bid switchgear providers continue to keenly excel in outdoing their peers in what they affectionately call “doors,” and particularly “hinges,” that are (surprise) wishfully intended to remain serviceable for 30 years.

 

[peebee]

Hey, just be happy the electrician put the door back on the last time he was in there.

 

[afterhrs]

Peebee,
I resemble and resent that statement!!!

 

[GusD]

Hi Hipotential

I don't know the theorectical reasons why bonding jumpers are needed other than to provide safety to people and protect equipment.As the previous threads indicate,it may be a lot of other reasons as well.
I have seen cabinet doors and transformer enclosures that show a distinct burnt mark around the enclosure bolt heads.At times these marks are in each bolt head on an enclosure.I do remember a time when we energized a new Power Tranformer and fire shot out around the bolt heads fastening the Secondary JB to the main transformer tub.
There were close to 50 , 3/8" bolts fastening the JB to the tub.Obviously added bonding was the answer. GusD

 

[bobbylove]

For what its worth, the military standard for Safety Design Criteria, MIL-HDBK-454A Guideline 1 recommends that hinges and slides not be used for grounding paths. Its common pratice in MIL systems to include a ground wire between the cabinet and the door. This is for personnel safety only, and not EMI/EMC as pointed out in an earlier post.

 

[jbartos]

Suggestion: My previous posting merely added to the previous postings that I essentially agreed with. The bonding doors; especially, when higher voltages are present is a prudent safety precaution, since the door may have higher potential against other conductive surfaces due to higher resistances in door hinges. Also, when contacted by live conductors, the door hinges may or may not necessarily trip the live conductor upstream protective device.

 

[Flywheel]

Underwriters Laboratory Standards UL 1778 - UPS Equipment para 19.4 and UL 1741 - Inverters, Converters, and Controllers for Use in Independent Power Systems para. 20.3 state " A metal-to-metal piano-type hinge is useable as a means of bonding a door for grounding."