To ground or not to ground a 2500 KVA, 480 volts diesel-generator

[piterpol]

Criterion that should be taken into account to decide what grounding method are to be used.

 

[dpc]

If you have a four wire system with 277 V loads, it must be solidly-grounded, or you must add a grounding transformer.

 

[arrehman1]

Take a look at IEEE Std C62.92.3™-2012 "IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part III—Generator Auxiliary Systems" This will give you some guidance as to your question "to ground or not to ground"

It is my understanding that it is much more common (now a day) to find industrial facilities with low resistance 4 wire system for better ground fault detection and isolation.

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[waross]

Regardless of the type of system grounding, all exposed non-current carrying metal parts MUST be grounded. That is the support frame, engine, generator end and control cabinet. Parts that are solidly bolted together such as the engine and the generator end may be grounded by one connection. Non-metalic motor mounts may require bonding jumpers. Many sets in that size range have the control cabinet rigidly and conductively mounted to the base frame. Often a ground connection to the base frame and a bonding jumper from the base frame to the generator end will suffice.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[piterpol]

Thank you very much for your useful replies.
One more question: If this generator were high-resistance grounded, ¿would it be prone to suffer overvoltage conditions?

 

[dpc]

One point - low resistance grounding must never be used for low voltage systems, e.g. 480 V. It's not safe.

High resistance grounded systems will see somewhat more overvoltage conditions than a solidly-grounded system, that is one of the trade-offs. But the overvoltage can be controlled, unlike ungrounded systems.

 

[piterpol]

Thanks dpc
You mentioned an important point: "But the overvoltage can be controlled". ¿May I ask you how?

 

[racookpe1978]

OK, so let me twist that answer ("grounding is essential") to a much smaller generator: 240 volts gas-powered portable unit on rubber wheels (no contact at all with the "grounded" ground!) at only 8600 watts?

Pound a stake in the dirt when the generator is running? Or just make sure I'm using a grounded 3-wire connection between load and the portable generator?

What about a light-cart/generator on rubber wheels at the job site? That's a larger, commercial generator-light rig. Do I assume the rental company has it properly grounded for 440 vac/120 vac loads out on the jobsite?

 

[MikeHalloran]

>> make sure I'm using a grounded 3-wire connection between load and the portable generator?<<
Yes, but that's not enough.

My 1.6kW/115/60 generator on rubber pads has a ground terminal on the frame, and came with manual verbiage and a bunch of yellow stickers, all stating unequivocally that the frame should be wired to a metal ground stake or a water pipe in the ground before even starting the engine. I store a roll of ~#10 ga wire with the generator for just that purpose.

That is the only system ground when the loads are connected by extension cords.






Mike Halloran
Pembroke Pines, FL, USA

 

[EP007]

I use a 5 A neutral grounding resistor for the generator on a 480 V industrial system. Lighting transformers step down to 120/208 V.

 

[ScottyUK]

racooke,

In British terminology, you're confusing the earth continuity conductor with the connection to the mass of earth. If you connect the metal cases together without a connection to the mass of earth then you only succeed in making them equipotential, but they are not earthed.

Substitute 'ground' for 'earth' as necessary!

 

[EP007]

ScottyUK: I was referring to the generator neutral grounding and the generator only. I wasn't referring to the bonding of the plant metal cases, nor to the earth electrode, nor the connection of the bonding conductor to the earth electrode.

 

[SparkyAt10]

A always prefer for generator output to be a 3-wire system for distribution and go for a digital high resistance ground system. The value engineering option is to delete the HRG and solidly ground the unit. This eliminates all the problems with ground fault protection when both sides of a transfer switch are grounded.

In a 4-wire system you have to decide if all your ATSs are 3-pole (solid neutral) or 4-pole (switched neutral), and make sure that any future changes to the system keep that same design philosophy. Next major upgrade to the facility someone will select the wrong kind of switch though, and cause circulating ground currents and potentially trip ground fault protection.