Is zone selective interlocking necessary on smaller systems? 2

[bdn2004]

We are installing a 1500 kVA, 4160V-480V high resistance grounded unit substation. The transformer was purposefully specified with a 7% impedence to reduce the fault current. The vendor has specified the switchgear with ZSI. We've ran the arc flash study and the energy levels are sufficiently low. Is ZSI overkill in this situation? It is a costly option.

 

[davidbeach]

Faster, coordinated, clearing is always a good thing.

 

[Eleceng01]

bdn2004,

I agree with davidbeach, but you haven't given enough info to argue your side - what is "sufficiently low"?

An arc flash accident can cost over 1-million in litigation, not to scare, but will your decision hold up in court?

Just something to think about. Hope it helps.

 

[bdn2004]

It's not that I am taking a side. The arc flash hazard is reduced per the modeling software down to a level category 0, 1 being the lowest 5 and beyond the highest. The switchgear has the proper short circuit rating. And we are installing insulated bus.

No disrespect intended here...but no one would ever be hurt in an auto crash if we all drove a tank. Safety is an important consideration - but at some point it becomes impractical or too costly a luxury to protect against an event that in all liklihood will never happen.

We are purchasing 7 of these units, and this one option is going to cost us close to $50,000. The switchgear it replaced lasted for more than fifty years without it. Would you spend your money on it?

 

[bdn2004]

oops I meant it's a catagory 2, 0 being the lowest, 5 the highest.

 

[PWR]

I would long and hard at spending the 50K.

First of all, presumably you are talking with ZSI you will have Category 0 on your switchgear bus? I find it hard to believe that selecting a main breaker with a short time setting of 0.1 sec is going to take you from zero to category 2.

As in an earlier comment, you have not given enough information for anyone to make an informed recommendation.

 

[bdn2004]

Maybe I am exposing my own ignorance here...but what other information do you need in order to come to a conclusion?

 

[PWR]

Issues to consider in making your decision:

ZSI allows you to have instantaneous tripping of the main breaker without, in theory, the risk of tripping it when a feeder breaker could have cleared the fault.

What if you skip the ZSI feature and just set the instantaneous on the main to trip at the highest setting that will still catch an arcing fault? How many times might it trip when it does not need to? If it does trip, what risks does that pose to the process?

Far and away the most common faults on 480V systems are ground faults and you have that covered with high resistance grounding. (For the latter you have to have a knowledgeable maintenance staff with a policy in place to actively seek out and remove faults after an alarm). Just how many "saves" over the course of the equipment's life will ZSI achieve?

If you don't allow instantaneous tripping with ZSI or otherwise, what is the true incident energy increase from a short time trip set at minimum delay? That will allow you to coordinate with instantaneous trip feeders without the expense (or complexity...I have seen many ZSI setups that don't work because of wiring or setting errors) of ZSI.

There are other issues concerning how well your facility implements safety procedures and enforcement of minimum PPE requirements and the like. How often will there be actual work performed in the zone protected by the main breaker,etc.

The Bottom line is that ZSI used on phase protection only achieves very little benefit for most installations in my opinion.

 

[dpc]

To answer your original question - Is it "necessary", clearly the answer is no.

But I would suggest you strongly consider including it. Arc-flash concerns are only going to become more significant in the future. Reduction from HRC #2 to HRC #0 or #1 does lower the risk as well as eliminate the requirement for a face shield.

BTW, what is the arc-flash level at the main breakers?

 

[Eleceng01]

bdn2004, I think that dpc's answer is one of the best (as seems to be the case with many of his responses)!

I was just trying to get you to think about the arc flash issue and provide more information.