Genset /Loadbank contactor and breaker help.

g.mccormick · Feb 24, 2015

We are building a testcell for testing diesel gensets. The maximum rating is 550KVA@400V.

There is going to be a loadbank used for the absorption of power with both resistive and reactive loading.

We are wanting to have a contactor inline so that genset power can be isolated from the loadbank (for voltage protection, etc).

We will also be wanting to put breaker infront of the contactor to serve as overcurrent protection for the wires as well as a disconnect/lockout point.

For sizing of the contactor and breaker, is 800A sufficent?
I am thinking of using the following contactor.
http://www.factorymation.com/Products/Metasol-Contactors-150-to-800A/MC-800A-22-AC-DC200.html

For breaker, a MCCB:
http://www.factorymation.com/Products/MCCB_800_Amp_High/TS800HU-FMU-LL-800.html

How does this seem?

Thanks

keithkyll · Feb 24, 2015

Contactor not rated for 400 volts. 550KVA is a lot more than 800 Amps.

g.mccormick · Feb 24, 2015

550kva@400volts ~ 795A

keithkyll · Feb 24, 2015

Okay. Sorry about that. (Didn't put 3 phase in the calculator

)
You should overrate to 125% for continuous loads.
"Read the rule closely; the 125% sizing of the OCPD (or 80% loading) is only applicable when continuous loads are involved. CBs and other OCPDs can be sized at 100% of their rating for noncontinuous load applications."
Link

g.mccormick · Feb 25, 2015

keithkyll said:
Okay. Sorry about that. (Didn't put 3 phase in the calculator )
You should overrate to 125% for continuous loads.
"Read the rule closely; the 125% sizing of the OCPD (or 80% loading) is only applicable when continuous loads are involved. CBs and other OCPDs can be sized at 100% of their rating for noncontinuous load applications."
Link

I would consider genset testing to be noncontinuous. Especially since the 550KVA set will only be part of the testing, and will only be 100% loaded very small part of the time.

Given that, I the the previously linked breaker and contactor will work.

shooter · Feb 25, 2015

rate the mcb to be 1600 amps as it should switch off reliable.
even when shorting under max load.
as you are testing.
the normal only needs 800 as it is abnormal but the testrig is normal for overload.

keithkyll · Feb 25, 2015

Higher voltage contactor.
I like this breaker from Automation Direct.

Shooter may be right about needing the higher ratings.

g.mccormick · Feb 25, 2015

keithkyll said:
Higher voltage contactor.
I like this breaker from Automation Direct.

Shooter may be right about needing the higher ratings.

The voltage is the coil voltage. I need the 220V coil.

That is a nice breaker.

g.mccormick · Feb 25, 2015

shooter said:
rate the mcb to be 1600 amps as it should switch off reliable.
even when shorting under max load.
as you are testing.
the normal only needs 800 as it is abnormal but the testrig is normal for overload.

I'm sorry, but I don't quite follow with what you have said here.

shooter · Feb 27, 2015

in normal operations 800 is good, as it needs to shut down only when an error occurs (that is maybe once a year, however on a testbank this occurs maybe even every minute so then the rating is not enough.
And be sure to use a safety circuit, as it must be reliable to shut down even in overload conditions, like 200%.

BobB · Feb 27, 2015

My answer is it depends! The generator could be tested for a number of hours. Also if it has been running on low loads for some time it will need to be run for 4-6 hours from time to time to stop/eliminate glazing of the cylinders. Additionally if load dumps are being made onto the generator during testing then inrush would be high and would need to be calculated into the breaker size.
I assume that the load bank will not be dumped onto the generator but stepped on? If it is dumped on it is highly probable that the generator will stall. You would have to che3ck with the manufacturer as to what the acceptable load steps are for the generator. Normally the first step should not be more than 40%.

shooter · Feb 28, 2015

there is a procedure for testing generators, one of them is over current break. if normal breaker does not work the safety breaker on the testset must be capable of breaking at much higher current.

Genset /Loadbank contactor and breaker help.

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