Bonding Generator housing structure that is bolted together

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PietjePuk

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Location
Calgary, Alberta, Canada
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Electrical Engineer
I have a question that I don't have fully answered and was wondering if you could provide some clarification or your insight on if at all possible.

I have a question on bonding/grounding:
  • Natural gas powered generator (5MW) inside a structure that encloses it, permanently anchored on a concrete pad on a site
  • Generator produces 13.8kV but the MV lines go directly from the generator to a switchgear (GCB, transformer etc) 6ft outside the skid
  • 480V source is brought in from the switchgear to power auxiliary loads (rad, ventilation, coolant pumps, oil pumps, airconditioning, heating,etc and other 120V auxiliaries such as lights, receptacles etc)
  • The structure is a base frame with bolted pieces to it to form the structure around it (bolted uprights, corners, etc).
  • Aluminum panel sections get bolted to the structure to form the walls and roof
  • All cables inside the structure are run in metalic trays along the sides of the structure
  • A bolted frame rad with motors and sensrs etc is bolted to the top
  • Some of the bolted frame pieces are 2ft in length
  • The trays and tray systems are all bonded and grounded.

I have done some searching, and compiled a short document with some, to me, controversial information.

My question is - does every bolted piece in the structure (enclosure and radiator structure) need to be bonded with methods indicated in document (bond strap, washers, clean unpainted surfaces), or only any pieces where there is a chance of it becoming energized by shorts/worn/broken cables? Obviously all control panels, cabinets, etc will have to be bonded as is common practise, but my question is more about the structure and wall panels of the enclosure and the radiator system on top.

I have never seen a structure where every pieces of metal in a frame structure is bonded.
 
These are the code references I looked at:

NFPA 79: Electrical Standard for Industrial Machinery

8.2.1.1 Grounding Circuit Stress. All parts of the equipment grounding (protective bonding) circuit shall be capable of withstanding the highest thermal and mechanical stress that can be caused by fault currents flowing in that part of the circuit. All exposed conductive parts of the electrical equipment and the machine(s) shall be connected to the equipment grounding (protective bonding) circuit.

Exception: Small parts such as screws, rivets, and nameplates that are not likely to become energized shall not be required to be grounded.



8.2.1.2 Equipment Grounding. The machine and all exposed, noncurrentcarrying conductive parts, material, and equipment likely to be energized shall be effectively grounded. Where electrical devices are mounted on metal mounting panels that are located within nonmetallic enclosures, the metal mounting panels shall be effectively grounded.

Where specified by the manufacturer, components and subassemblies shall be bonded to the equipment grounding (protective bonding) circuit in accordance with the manufacturer’s instructions.



8.2.3.3 Bonding of equipment with bolts or other identified means shall be permitted where

paint and dirt are removed from the joint surfaces or where the bonded members are

effectively penetrated.
 
I've been on some large installations where the cooling was a heat exchanger mounted separately (outside) and I I don't remember if they were bonded anymore then the EGC and even if it was bonded with something heavy I'm sure Cat required it.

But in that case I can see a need for a bonding strap
 
I don't see a generator as being within the scope of NFPA 79.

Even if it is, the parts you are talking about are not likely to become energized and would not require bonding.
 
Some generator manufacturers specs call for a bonding strap between a generator engine and whatever kind of heat exchanger you are using to reduce electrolysis.

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That's one we sat on a roof down by Camden NJ. Thinking back that was like 2006 or 2007. Did a few them back then. But I was out of there once it was in place.

A lot of Cat engines suffer from what is known as "Water in the basement" meaning that because they are a wet sleeve design, there is a failure of the sealing rings at the bottom of the cylinder liner. It really sucks because the engine can still run great, pull full power on a load bank, and yet there is coolant showing up in the oil samples, shortly followed by milky oil if they keep using it for any amount of time.

So anyway, you have to keep up with the cooling system maintenance both chemically and protection against electrolysis
 
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