Different grounding of two separately derived systems

[deltawhy]

Hello,

I'm wondering if anyone can point out any obvious drawbacks to having a normal source (utility) solidly grounded (3P 4W system), with an emergency system (separately derived) impedance grounded. All circuit breakers within the main and emergency distribution would be 4 pole. There would be no single phase loads on the bus.

The reason the utility source would be solidly grounded is the utility owns the transformers and will not allow for resistance grounding. This is in Ontario, Canada.

 

[waross]

The voltage makes a difference.
If the voltage to ground will be less than 150 Volts than the system must be grounded.
Most of the sets that I have installed or repaired have been 120/208 Volts or 120/240 Volts that would be grounded under Canadian regulations.
I have installed, repaired and worked near 277/480 Volt sets that were all solidly grounded.
My preference is to connect the set neutral solidly to the service panel so that it shares the normal system ground.
Disadvantages:
Four pole transfer switches are more expensive to buy and to install.
I have more than once seen damage caused by a switched neutral failing open or switching late.
That was damage that would not have occurred with a solid neutral and a three pole transfer switch.
Some designers like separate grounds on generators and four pole switches.
I don't.
My preference is a solid neutral and let the generator share the system ground, be it solid or impedance grounded.
I hate four pole transfer switches.
I would rather avoid dealing with customers who have just lost a lot of equipment because someone else used an el-cheapo four pole switch.
The customers are mad and not very trusting.
That said I admit that there may come a day when a generator is some distance remote from the ATS and main service and an exception to my general rule may be warranted.
Hasn't happened yet but it may.



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

 

[Kiribanda]

1)When the load is connected to Utility solidly grounded system,the phase voltages of the healthy phases
will be not more than 80% during a single line to ground fault.As a result the voltage rating of your MCCBs
(I am sure that they are slash rated MCCBs) are acceptable.
2)When the load is connected to the impedance grounded emergency system, then the phase voltages of the healthy phases
may be equal to the full line to line voltage during a single line to ground fault.As a result the voltage rating
of your MCCBs SHALL be non-slash rated MCCBs.Otherwise, when there is a second ground fault in the system,the
single pole interrupting capability of slash rated MCCBs will be compromised.
Therefore,I am in the opinion that it is advisable to use solidly grounded system when the load is fed by the emergency system too.

 

[deltawhy]

Thanks for the replies.

The system is 600V, and proposed generation would be up to 5MVA (multiple parallel units), so the LG current would be quite significant.

No breakers would be spec'd as slant rated. However, a 4 pole ATS would be needed to separate the grounding systems.

Given this, would there be any possible code violations anyone can think of for two types of grounding (while the two systems are separately derived)?

 

[waross]

With generators, too little fault current is often more of a concern than too much fault current.
I would compare the fault current from the utility source with the fault current from the generators.
I suspect that the generator fault current will be less than the utility fault current.
If the generator available short circuit is comparable or less than the utility fault current then I would not worry about a solid ground on the generators.
If you are using any 347 Volt lighting directly from the utility feed than you may need to solidly ground the neutral.
In the older codes that would be MUST ground the neutral, but there may have been changes.
Be aware that with a resistance grounded system, in the event of a persistent ground on one phase, your electricians may encounter an unexpected 347 Volts on the neutral when trouble shooting.

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

 

[dpc]

I assume you're aware you won't be able to have any line-to-neutral loads. I think this would be acceptable in the US with the 4-pole transfer switch, although I don't recall ever seeing it done before. No idea about Canada.

 

[waross]

This was the law in Canada in the 2015 Canadian Electrical Code as adopted by the various jurisdictions.
10-106 AC systems
(1) Except as otherwise provided for in this Code, ac systems shall be grounded if
(a) by so doing, their maximum voltage-to-ground does not exceed 150 V; or
(b) the system incorporates a neutral conductor.
(2) Wiring systems supplied by an ungrounded supply shall be equipped with a suitable ground fault detection
device to indicate the presence of a ground fault.
(3) Ground fault indication activated by the ground fault detection device required by Subrule (2) shall be
clearly
(a) labelled as to its purpose; and
(b) visible to persons monitoring the status of the system.

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