Safety of a separate bonding jumper for exterior equipment?

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greenspark1

Senior Member
Location
New England
Hi. I have a commercial building with a large outdoor motor on a concrete pad. The facility is properly grounded and has a ground ring around the whole building. The motor is powered by three phases and has a wire type EGC. The concrete pad of the equipment has its own bonding jumper that ties into the ground ring.

My concern is that this bonding jumper for the concrete pad creates a parallel path for ground current to flow. This means that ground current will flow on the ground ring back to the service ground, ultimately bypassing the building's electrical system and not tripping any ground fault breakers. While adding a ground for this equipment pad might seem like a good idea, I feel that it actually compromises the electrical safety of the facility. I would prefer for the ONLY ground to be through the EGC.

This would be especially true if there was a concrete pad for a non-electrical piece of equipment, like a plastic tank. Bonding the pad will completely bypass the electrical system.

Thoughts & comments?
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Hi. I have a commercial building with a large outdoor motor on a concrete pad. The facility is properly grounded and has a ground ring around the whole building. The motor is powered by three phases and has a wire type EGC. The concrete pad of the equipment has its own bonding jumper that ties into the ground ring.

My concern is that this bonding jumper for the concrete pad creates a parallel path for ground current to flow. This means that ground current will flow on the ground ring back to the service ground, ultimately bypassing the building's electrical system and not tripping any ground fault breakers. While adding a ground for this equipment pad might seem like a good idea, I feel that it actually compromises the electrical safety of the facility. I would prefer for the ONLY ground to be through the EGC.

This would be especially true if there was a concrete pad for a non-electrical piece of equipment, like a plastic tank. Bonding the pad will completely bypass the electrical system. ...

Green -
I'm thinking you already know all of this.

Ground fault protection measures the current going out on the phase (and neutral) conductors. Summation I = zero - Or there is a ground fault. The fault current will travel back through any path to the N-G bond back to the source. Regardless of the number of paths, the GF protection is not compromised.

Looking at the motor: The motor pad steel is always connected to the motor steel frame. The idea is to keep the pad and the frame at the same potential. If the motor faults, the frame can be raised to line potential until the OC (or GF) protection opens.

The primary GF current path back to the source is through the motor EBC. If there is a second path from the motor pad steel back to the service, that will carry current as well. But that won't change the GF protection.

Note: A second dedicated EBC from the motor pad steel is sufficiently rare that I've never seen one. However, motor pad steel connected to a ground grid is normal in industrial installations.. The motor pad steel will still be connected to the motor frame.

And I'm still thinking you know all this

ice
 

greenspark1

Senior Member
Location
New England
Good explanation iceworm, I admit it was somewhat of a senior moment on my part. However, I believe I have seen ground fault systems that use a CT on the ground bus to trip the main breaker in case of GF. If this were the case, additional bonding jumpers would be a problem since current would bypass the main ground bus.

Especially working on existing facilities, I don't always know the details of how their GF protection is set up.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Good explanation iceworm, I admit it was somewhat of a senior moment on my part. However, I believe I have seen ground fault systems that use a CT on the ground bus to trip the main breaker in case of GF. If this were the case, additional bonding jumpers would be a problem since current would bypass the main ground bus.

Especially working on existing facilities, I don't always know the details of how their GF protection is set up.

No, additional bonding jumpers are not a problem. If the system is installed incorrectly - it's not going to work correctly. Additional bonding jumpers don't change that.

As for not knowing the details of existing installations - that's always true. You don't - never will. Additional bonding jumpers don't change that either.

Consider a normal industrial practice of using a ground mat under the entire facility. All pad steel will be connected to the ground mat. All pad steel will be connected to the motors. How is this different from the additional bonding jumper? Would you consider this practice bad?

I'm not seeing the issue.

ice
 
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