Generator plug vs. hard wiring

[paulengr]

Other than trying to enforce the difference between temporary and hard wiring, is there any technical purpose behind the rule requiring cord-and-plug connections on portable or vehicle mounted generators without a ground rod?

Reason being that the particular instance is in a large mining operation. We use portable cords extensively (up to 300 MCM/7200 VAC). We have lots of "small" 30 HP or 50 HP temporary dewatering pumps. Although plugs are more convenient the general tendency with the electrical group is to hardwire the cords due to reliability issues with the plugs (corrosion and water).

It seems relatively silly that the only difference between wiring up a portable generator with a 30-50 HP pump attached and requiring a ground rod is whether or not it has a plug connection.

Ground rods are a bit of a problem because the soil on the mining pit floor has very poor conductivity. The vast majority of the power distribution outside of generator-supplied uses high resistance grounding systems with grounding grids established at the substations. Everything in the pit is grounded via 4-wire wye connections back to the subs.

 

[ceb58]

Other than trying to enforce the difference between temporary and hard wiring, is there any technical purpose behind the rule requiring cord-and-plug connections on portable or vehicle mounted generators without a ground rod?

Reason being that the particular instance is in a large mining operation. We use portable cords extensively (up to 300 MCM/7200 VAC). We have lots of "small" 30 HP or 50 HP temporary dewatering pumps. Although plugs are more convenient the general tendency with the electrical group is to hardwire the cords due to reliability issues with the plugs (corrosion and water).

It seems relatively silly that the only difference between wiring up a portable generator with a 30-50 HP pump attached and requiring a ground rod is whether or not it has a plug connection.

Ground rods are a bit of a problem because the soil on the mining pit floor has very poor conductivity. The vast majority of the power distribution outside of generator-supplied uses high resistance grounding systems with grounding grids established at the substations. Everything in the pit is grounded via 4-wire wye connections back to the subs.

I think the main difference is with a cord plugged into the gen set. the ground and neutral are tied together making it a separately derived system. If you were to hard wire into the set,as a back up to power something like a building, then the grounds and neutral would then need to be separated and a ground rod requried. Look over 250.34 B

 

[petersonra]

Ground rods are a bit of a problem because the soil on the mining pit floor has very poor conductivity.

How is the soil conductivity an issue at all? Pound two rods and be done with it if you want to hardwire.

 

[paulengr]

How is the soil conductivity an issue at all? Pound two rods and be done with it if you want to hardwire.

I can't believe you are actually saying this unless you never actually check to see if you have anything approaching a decent ground grid in the first place.

In an MSHA regulated area, routine checking/verification of your ground is required, at least annually. There is no way that pounding two ground rods is going to get you even close to 25 ohms let alone 15 ohms (our internal requirement). At a minimum in areas along the top of the high wall outside of the pit with undisturbed soil near a water source, we usually need a minimum of 3 ground rods wired in parallel. Often even that isn't enough. And dumping bags of salt on them and watering it in isn't going to work because a few weeks later you'll be in doing it again.

So we use threaded rods, multiple ground rods, and get it down reasonable at the 25 kV substations at the rim where we have a fighting chance of achieving a decent ground grid. That's on top of having a high resistance ground which requires substations to have two separate ground grids as a minimum (safety ground and system ground).

 

[RETRAINDAILY]

And not having too buy a ATS.

 

[petersonra]

I can't believe you are actually saying this unless you never actually check to see if you have anything approaching a decent ground grid in the first place.

Why do you care about the ground connection for a portable generator running a 50HP or smaller motor? What does a "better" ground do for you?