How to caclulate Short Circuit Impedance For A Industrial Plant .

I recall a distributor mentioning a new product they have, high speed relaying or something solid state, that they use for tough arc flash applications. He used 600V MCCs as the example. They knocked 100cal/cm^2 down to 8cal/cm^2 in one instance.

So far I haven't had to do any arc flash calculations. What are the dominant factors in a calculation? I? I^2t? SCMVA? V? Got any advice to share for situations that look like a high cal/cm^2 value is likely?

Sorry, don't have anything to share about mitigating it. What distributor product were you looking at?? When would such a product be used instead of just concentrating on fault prevention?

Mostly been dealing with the new safety training and PPE (personal protective equipment) needed by OSHA on this issue.

daestrom

Well, I do, but on the other end of the problem. We have three utilities in my stomping grounds. One has a networked secondary system downtown, which has grown over the years, leaving older customers with woefully under rated service equipment.

All of them have primary service customers and don't seem to consider it a problem when they build a substation just across the street from one and don't tell them their fault current just went up.

So I get a job to add a feeder and breaker to the main panel and I've got to tell them that the whole thing has got to come out. Even suggesting CL fuses cause them to wail and clutch their wallets.

You may have some limited control over the working distance, but you can't change the system voltage. The two factors that you have the most control over are the available bolted fault current, and the clearing time of the protective device. Higher fault current can sometimes reduce the arc hazard by decreasing the clearing time for a given protective device. You need to vary the parameters and see what the results are.

Look at splitting circuits into multiple smaller transformers and feeders instead of a single large one, faster tripping protective devices such as current-limiting fuses, etc.

Ben Miller

This is part of the reason why the utility here forces installation to high fault levels - 'future proofing'. I wonder how often people are hurt or property damaged due to having devices w/ interrupt ratings lower than available fault levels. If it's very rare then it's good if people aren't forced to spend huge cash to protect against a relatively lesser threat.

The distributor was EECOL Electric. I don't think they exist in the States. I don't know what product he was referring to, or what manufacturer made it. I just filed it away in my noggin for the day that I'm dealing with the issue. If I'm talking to the guy again, I'll find out a little more.

When would such a product be used instead of

When you don't want to (a) buy and maintain ridiculous protective gear and (b) have electricians forced to work in ridiculous protective gear, you might find it preferable to reduce the arc flash hazard. I am not yet involved in arc flash protection to any extent so I can't comment on 'fault prevention' as it relates to arc flash hazard.

Sounds fun.

I've seen one good one which may or not be relevant.

About 25 years ago I was doing some unrelated work in a 1910-era building in the downtown area of Lockport, NY. I happened into the basement and saw what looked like a large demolition project, bricks and tile and mprtar in piles all over the floor. Also saw some brand new 208 switchgear arrangeed as a temporary service.

Turns out the original service, lead and paper cables, faulted, and the "demolition" I saw was the result! It completely blew apart the (indoor) service vault. No injuries but significant structural damage.

Investigation reveals that prior to the accident the building's service had NEVER been upgraded. Simply bus bars around the walls of the vault with fuses tapped onto the bus. NO service disconnect, not even switches upstream of the fuses.

A few years later I saw a smaller version of the same thing in a building down the street. Have pictures somewhere.

Future-proofing is fine if you know where system growth might occur. If not, you are imposing higher costs on customers who may never require the increased fault duty ratings.

On the other hand, the NEC is pretty conservative and doesn't allow for probability of failure modes and cost benefit analysis to justify forgoing adequate system ratings.

Problems arise when one must inform a customer that the service which has performed adequately for years must be upgraded at significant expense. Utilities are motivated to overestimate future fault duty requirements until they run into customer complaints. Why should a customer pay now for something that may not be required for years, or possibly never? The problem is that there is no mechanism by which a customer may install the minimum cost system required, but be required to upgrade when the utility system is upgraded. This would be something that a customer (large commercial or industrial) would agree to in exchange for receiving a lower initial design fault current.