Thermal Damage Generators/Motors

Thanks for your reply and I am quite certain the last paragraph was accurate!

There is only short circuit which causes to increase heat in any electrical equipment, you need to study the short circuit and the max. heat temp.

I would recommend you these files to better understand,

Short-Circuit Effects

Alternator protection

Thank you as it was helpful. I did have the Cummins white papers which were also helpful.

I'm just not certain why it isn't standard procedure for gen. manufacturers to supply this info with their short circuit data. Hopefully I'll find out tomorrow by calling them but it would be nice to be able to plot thermal damage curves for short time pickup circuit breaker settings!

Thermal damage curves? That's a new one on me.

Well, it's only because the Isc is only there for sufficient time for the circuit protective device to disconnect it from the source of energy, Boss. After all, what individual of a sound mind would want to run a generator into a short circuit for any longer than this period, eh?

Mr. Crabtree, thermal damage to rotating rapt. happens way before short circuits normally around 3 x Fla. After researching on the internet I've found few facts except it was said to be an afterthought and these engineers are a dying breed! I spent most of the weekend coming up with that and let's face it, there are some damned good engineers on this forum and I haven't received an answer yet! Thank you for your response.

No, you haven't received an answer you like yet. You have received several answers. There is a difference.

Now if you explained why you did not like the answers you've been given or if you explained why you think an answer that meets your criteria should exist, then you might actually get somewhere.

You also should stop using abbreviations and acronyms without defining them. I have found many times that once one stops using jargon and buzzwords that the actual root of a problem becomes clear.

Yes. I like that. GA.

jraubsr,

I'm glad you are studying this topic. Perhaps a search on "fault current" would give further education. As a senior electrician working in the service department of a contracting firm, I have had to work with this a number of times. I_sc, the short circuit current, is a totally different topic than the thermal damage curve of a generator set. Both are important, but for different areas of concern. The thermal damage curve is looking at the effects of any type overload on the internal components of the generator (alternator) set. The good technical paper linked by another commentor gives a good discussion of how this relates to the operation of circuit breakers and other protective devices.

However, the I_sc is the beginning point for a calculation of the external risk of damage (injury or worse) primarily to people, and also peripherally to equipment. The calculations involved all look downstream to where people can be exposed to short circuit faults and the arc blast energy they contain. NFPA 70E (The National Electrical Safety Code) is the primary reference for this. There are a number of numbers (data) that you need in order to calculate the thermal energy of an arc blast under the assumed fault conditions. The two most important ones are the fault current (I_sc) at the fault and the time duration (either until an upstream protective device opens or the people have been able to move away, whichever is first). The other numbers are the voltage, the distance from the fault to the worker, and the surroundings (open, in a box). When you know the I_sc, you can look at time/current curves for the protective device and come up with the time value. Important to this is to consider that a possible fault may have a lower fault current, so good calculations will look at the energy for different fault currents along with the time for clearing these lower currents (I have seen cases where the lower current has a much longer clearing time and therefore a significantly higher total fault energy).

The end result of such a fault analysis will be to tell the energy available at any fault and thus prescribe the level of personal protective equipment needed to even open an enclosure door! The danger in arc flashes and arc blasts must not be ignored. A friend of mine spent days in the hospital after a screwdriver shorted out a live busbar in an MCC he was working on.

I have seen the calculations of available fault energy at a large industrial plant. In two locations, the available fault energy was greater than 40cal/sq.cm. (the level at which no personal protective equipment is adequate)--at the main utility service entrance and at the disconnects where 1500kVA generators were connected to substations. These points were hundreds of feet apart, and at equipment between them, the calculated available fault energy levels were much lower.

--John M.

An excellent answer.