# Generator Dead Bus Test



## nrp3

I have a small load bank that I use from time to time to see whether an existing system can support its rated load. I'm talking residential and light commercial in this case. If it can't then we start looking at whether the gas piping was sized correctly or maybe due to age something like a bad ignition, or other things. On the large scale, I've read about the use of load banks to properly exercise diesel units with load to keep exhaust systems clean.


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## Big John

Yes, load-banking is a common test that we recommend to all customers who rely on generator for standby power.

Have seen and been involved with a number of serious generator failures despite the fact that the gens were routinely exercised.

Putting the generator under worst-case expected load for long term causes much more electrical and mechanical strain than simply spinning it up for a half-hour a week. It is far more likely to fail in service, and that's what load banking helps flush out.


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## drsparky

Post Glover make dynamic braking resistor for slowing down big motors, they probably make a load bank that would fit your generator.


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## nrp3

They have permanently mounted units and portable ones as well. Mine is in a big carrying case about the size of a large PC. Exercise is good, knowing it'll carry the load it's designed to is better. The larger the unit, the more complex the procedure of course.

Mosebach (I think I spelled that right), and Avtron come to mind. Google will bring all kinds of options up.


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## Jackofmany

A dead bus test checks the generators ability to close on a bus with no voltage on it, hence "Dead Bus". This is not a load test using a load bank. This test is usually performed on generators that are part of a paralleling system which has controls to match voltage, frequency, and synchronization. When the bus is dead there's no voltage to match to so one generator must have the ability to close on the dead bus establishing the voltage and such on its own.


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## brian john

glen1971 said:


> I'm not sure I've heard of this test before, or maybe it was something different... This is on an existing installation as a PM...
> 
> The test I'm thinking of would be connecting a resistor bank to the generator, instead of the usual load, and completing a load test on the generator, as performance test.. I've only seen these done on new installations..
> 
> Has anyone heard of or done one before? What is entailed?


I did two 3-megs and a 2-meg last week.

We do this all the time.

Bring load bank to job site.
Size cable for load test, 
Lay out cable
Connect to dead bus and isolate gens from operation to loads.
Perform a step load to what ever percentage the end user requires.


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## nrp3

And how do these paralleled units come on line, in sequence, all at once?


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## Jackofmany

In large systems with multiple generators the first one to start and establish proper voltage and frequency will close onto the bus when the other generators match they will be allowed to close their breakers. In a single generator system with an automatic transfer switch there's no bus monitoring needed due to the transfer switch detecting the loss of line voltage, starting the generator then transferring the load when voltage and frequency are correct.


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## glen1971

brian john said:


> I did two 3-megs and a 2-meg last week.
> 
> We do this all the time.
> 
> Bring load bank to job site.
> Size cable for load test,
> Lay out cable
> Connect to dead bus and isolate gens from operation to loads.
> Perform a step load to what ever percentage the end user requires.


 
Thanks Brian! That sounds like what I was picturing, just wasn't sure of the name of it. I was asked about how long it would take and I guess-ta-mated about a day for each site, without knowing their duration, including set-up and demob time... These are about 500 kw 480 volt gens I think...


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## Big John

If this is an emergency or standby system, check out NFPA 110 for guidance on how to do these tests. If it's full load, it will be a 2 hour test.


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## glen1971

Big John said:


> If this is an emergency or standby system, check out NFPA 110 for guidance on how to do these tests. If it's full load, it will be a 2 hour test.


Emergency generator on a sour gas compressor... Non sync... Been in service since about 1978, with regular maintenance and goes online about 12-15 times a year... Gets started monthly and ran with no load..


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## glen1971

Jackofmany said:


> In large systems with multiple generators the first one to start and establish proper voltage and frequency will close onto the bus when the other generators match they will be allowed to close their breakers. In a single generator system with an automatic transfer switch there's no bus monitoring needed due to the transfer switch detecting the loss of line voltage, starting the generator then transferring the load when voltage and frequency are correct.


Some will synchronize back to the utility upon power restoration but most are an open transition (break before make) and will isolate the load before making the transfer either way..


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## Big John

So, what's your job scope do you have to test black-start or full-load carrying?

If it gets loaded a dozen times a year it seems a load-bank wouldn't really be necessary.


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## glen1971

Big John said:


> So, what's your job scope do you have to test black-start or full-load carrying?
> 
> If it gets loaded a dozen times a year it seems a load-bank wouldn't really be necessary.


That's why I was scratchin my head, as to why do this test and exactly what they are looking for.. 

So far I'm thinkin I'm overseeing a sub contractor do it... The boss was curious if we could do it online, and how long.. I said you could do it online, but if we lost power, you'd be offline pretty quick as the generator would be disconnected..


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## brian john

Jackofmany said:


> In large systems with multiple generators the first one to start and establish proper voltage and frequency will close onto the bus when the other generators match they will be allowed to close their breakers. In a single generator system with an automatic transfer switch there's no bus monitoring needed due to the transfer switch detecting the loss of line voltage, starting the generator then transferring the load when voltage and frequency are correct.


We have sites first up, first on line other sites they set the unit to take the load first. 

Some sites do closed transition some parallel with the utility for longer, most are open transition. .


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