Emergency generators

A facility supplied by 2 of 2250kva 13.8kv/0.6kv transformer from utility with tie breaker NO. There are 2 of 1000kw, 600V emergency generators (generator tie braeker NO) supplying essential equipment in the system in case of utility power failure. Customer says the generators are rarely used, and no need to run emergency generator operating scenario for arc flash analysis. My question: By codes, do we have to run emergency generator (only be in operation in emergency situations) operating scenario for arc flash study? I understand that we can forget about small generators because of low SC. What will be a proper generator capacity cut-off line for arc flash study?

Thanks all

J.T
Techtric engineering

Interesting question.

First, generator are the worse case for arc flash because of the low fault current. That means long clearing times and high IE numbers (typically).

Arc Flash deals with live work or work on enclosures that switch significant energy levels. So if you prohibited all work while on generator you could make a claim that an arc flash study is not required. But that would have to be well documented and in the training.

NEC 110.16 still require arc flash warning labels and I don't see a way around that.

I would recommend that you do the analysis and see where it comes out. There is a good chance it will be high but you need to know. Even if you prohibit the work while on generator you should have the analysis as back up.

Hello,

In my organization we have Operating Orders that dictate how equipment is switched or used in special conditions. For the equipment that has been deemed Extremely Dangerous because of Arc Flash Hazard, those SOO's basically say "De-energize, do not work live". Arc flash related SOO's are noted on the labels.

Maybe you could label to the worst case hazard but then reference a standing order that allowed live work under certain conditions described in the order.

Regards

Arc flash scenarios

While I might agree with the customer, I was in the exact same situation with my facility. Our generators are rarely used (2 X 1000 KVA @ 600 V + 1 X 1000 KVA @ 13.8 KV). My consultant asked the same question of me...
The generators must be included in the overall study because of potential for long clearing times and high incident energies. An additional label is required for Standby Power no matter how infrequently the generators run. The customer can then determine safe working procedures based on your study. Anything less can put the workers at risk - the facility is on standby power for a reason. On Standby Power, electricians have a right to know the system went from relatively benign to outright dangerous!
As far as sizing - 125 KVA generator is probably the cutoff??

We always plan on running a normal utility case and a standby generator case when preparing arc flash hazard analysis because we want to know the short circuit contribution for coordination purposes and then the incident energy available in that mode of operation.

Run off emergency generators is a rare occurrence, and at that time, there is less likelyhood for performing live work. The IE values from the generators are generally very high because of the long clearing times. I've dealt with it in policy and labelling, by stating No Live Work can be performed will on emergency power. When the generator is running, the odds are you are working on the dead bus that just tripped. There is limited reasons why you need to work on the system live when on generator.

Many of our clients with multiple parallel connected U.P.S. systems will regularly turn on the generators if they are working downstream to the U.P.S. system.If the U.P.S. system trips for any reason it will go to static bypass fed by these genys.

Have the generators off and let the static bypass go to the utility instead. You're not on a ship, so obviously most of the time you are running off the utility. If you have to work live with the generators, then do the calcs but they will come out high, unless you want to change the mains to ones that have a full sprectrum of trips curves but especially with Inst Pickups that can be set to a value down to 3 or so. If you can get the breaker to trip fast with the limited fault current, you will have low IE's.

Our major clients are involved in the banking/stock exchange areas.They NEVER use utility power to directly feed their computer servers etc.The added failure risk is WAY TOO HIGH and will affect their 9's of reliabilty.In this situation, the paralleled geny's(usually 3-4 at 1250 KVA. each) and supporting loads will have arc flash calculations performed.

Isn't the goal to find the worse case scenario?

Realistically speaking you may be more apt to be doing work when generators are on, say you have a failure at the main sub yard, etc.

I typically do studies of all scenarios and include them in my report basing the Arc Flash labels off the worse case.

Further more you are going to want to make sure your OCPD have adequate AIC ratings especially on the gen bus. Now if the gens and utility are closed transition or parallel I would say it is very important to include.

Finally the client hired you for a reason, you know what you are doing (or at least you are supposed to)

Just my 2 cents.

edit-

I'm mainly talking about large generators and/or medium voltage ones as for cut off I suppose you could use the same as a transformer. But it wouldn't hurt to model it anyways.