Maintenance Practice on MV Contactors

[BECREZ]

I have a question to which I hope one of you can provide insight.
I have a U.S. client who wants to know how to ground the MV cables in the MV contactor compartments so he can comply with the NESC when he does maintenance on the MV motors.
For Switchgear, we have the ground and test devices which can be used. What is the procedure for the MV contactor sections? Some kind of clamp to the load side stabs?

 

[jraef]

I have not read that standard, but I don't interpret that as meaning they have to be continuously grounded during maintenance if the controller is disconnected and locked out. Typically the requirement is that the cables are connected briefly to ground in order to discharge any capacitive charge left on them from being energized last. Most MV electricians I have watched do it with a hot stick holding a cable clamped to the ground bus and just move the cable from load terminal to load terminal on the contactor.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[swn1]

In practice, most US operators do not ground motor leads when doing maintenance. We're lucky if they even know how to verify the disconnecting means is open.

Equipment varies so much, and most of it does not provide a safe place to hang grounds.

Momentarily grounding the conductors does little for you. It momenarily discharges any static charge, but the motor windings do that too. A proper maintenance ground has to be able to cause the protective device upstream to operate if the circuit is accidentally energized.

There is a product you can install that will help, but you may have to provide access doors to use them. It's a bolt with a big round head that you attact to the bus bar, cable lug joint, whatever. They come with plastic covers that snap on/off of the ball with a hot stick. You also get a ground cable set with special clamps that grip the balls. Tape the joint leaving the balls exposed and you can ground the system safely.

But again, in the typical MV motor starter arrangement in service today, there is no safe place to put the grounding lugs.

If the starter is a drawout type you can have a switchgear service company build a "grounding truck" on a scrap contactor frame. Rack out the starter and rack in the grounding truck to ground the load stabs. But that may require putting the disconnect operator in the connected position, depending on the racking mechanism and shuttering arrangement.

There is no good retrofit solution. You may have to dedicate extra floor space to provide a junction box for grounding access if regulations absolutely require it.