Using utility breaker for arc flash protection 1

[PowerDawg]

I represent a utility that serves a chip mill. The chip mill’s MCC room is served by a 1500 kVA, 4160/480V transformer. The MCC room has no main disconnect, just individual disconnects serving various equipment. The chip mill is asking us if they can install a protection relay in their MCC room and use it to trip our substation breaker due to arc flash protection concerns. The chip mill is the only customer on that breaker. This seems to be the cheapest way to protect their employees from arc flash since they do not have a single source main breaker in their MCC room. I, however, have concerns about them relying on our station breaker for their protection. It seems to me that I would be putting our company at a liability risk by allowing the chip mill to trip our breaker should something go wrong and our breaker fail to operate. Is it reasonable for them to ask to trip our breaker or should I just tell them they will have to install their own main breaker to use for their own protection?

 

[cranky108]

I would not allow this either, as it is a liability risk. Tell them you will run it by your legal group, and a year later they will buy a main breaker.

 

[ZeroSeq]

I second that, after dealing with utilities on this exact issue, the customer should be responsible for their own protection. Definite inherent liability.

 

[dpc]

the customer should be responsible for their own protection.

Even if they install their own main breaker, someone working on or around the main breaker will still be dependent on the utility's primary side device for protection - that is just the nature of services.

But I agree that the customer should install their own main breaker and make that the new service disconnect - preferably remote from the existing main board. Reliance on transfer tripping of a utility-side breaker creates concerns on both sides of the meter. We have also had industrial customer install their own primary protection (reclosers), after buying the transformer from the utility. They are using transfer trip on these reclosers for arc-flash reduction.

 

[bacon4life]

Currently, if there is an arc flash, is it going to be your utility breaker that clears the fault? They are already relying on your breaker to trip. Adding an arc flash signal just adds the incremental liability for tripping faster rather than slower.

We generally have more confidence that utility equipment will operate properly than customer gear. We used to allow customers to own the overcurrent protection device, but changed practices after discovering a customer had removed the batteries from a switchgear lineup. We generally allow customer to provide a trip signal to our breaker.

 

[jghrist]

We have a municipal client with a 12.5 kV substation served by a utility from their 115-12.5 kV substation. The main 12.5 kV breaker is in the utility substation. We recently added a fast bus trip scheme that uses 12.5 kV feeder relay blocking signals (mirrored bits through fiber) with the utility instantaneous relays to trip the main breaker with only a 3 cycle delay. One reason for adding this scheme was to reduce arc flash incident energy.

 

[davidbeach]

Speaking from the utility side, I would never consider accepting a transfer trip signal from a distribution customer that didn't have a dedicated utility main that we could verify settings and periodically test. Probably not even then. Take service at transmission and the we'll talk. You need to provide your own device, sorry.

 

[stevenal]

What happens when an alternate path is used? Or the substation feeder breaker is bypassed?

 

[wbd]

Having worked in a utility for 30 yrs, I know that having a transfer trip from a customer for personnel safety ( arc flash) would not be allowed. However now being on the other side of the fence so to speak as a consultant performing electrical studies I have requested utilities to make changes to the protective devices. This has been mainly changes to riser fuses or recloser settings where the recloser only feeds the customer. Some have been for arc flash other changes to coordinate with the utility protective device.

So for this case, the substation breaker feeds directly to the customers 1500kVA txf? Seems odd that there are no primary fuses on transformer and this is a relatively small transformer in the big power world.

 

[PowerDawg]

Update... now the customer is asking if we can support S&C SMU-20 type fuses at our station. If we did that, I would remove the breaker all together and install the fuses only. But now the question would be if we are any more or less subject to liability with fuses as we would be allowing the customer to trip our breaker?

 

[ZeroSeq]

Many time I have asked the utiliy(ies) to change or alter their primary protection, whether it be a fuse change or relay time dial setting. Half will comply and half will not. I think each utility views their liability differently when intentionally changing their own standard protection schemes to conform to the customer's wishes specifically for AFH reduction. I've never had an arc flash on the line side of the incoming gear to test this theory from a legal standpoint, so I can't help you here.

 

[PowerDawg]

After consideration, the fuse idea is really no different that letting the chip mill trip our breaker... we are still accepting part of the responsibility (and thus part of the liability) for proper operation and design as it relates to the reduction of arc flash at the customer's equipment. With that said, our response to the customer is "no" to tripping our breaker and "no" to installing fuses. The chip mill needs to solve their own issues without relying on the utility to do it for them and exposing the utility to additional liability. I really hate it... wish we could help, but this is the litigious society we live in now.

 

[ScottyUK]

Wonder if your customer has considered a fault-thrower type arc suppression unit such as this one from Eaton?

It would (hopefully) trip the upstream breaker, but the arc would be gone by that point. Things could get a little exciting if the upstream breaker failed to clear and fault clearance was by backup protection - not sure I like the idea but offering it to provoke discussion. Has anyone used these and had one operate 'for real'?

 

[stevenal]

Perhaps the old concept of the high speed grounding switch could be dusted off. Low side relay triggers a high speed grounding switch on the high side. Arc is quenched and whatever protection is employed upstream by the utility clears the fault quickly. This wouldn't need any utility inter-tripping.

 

[cranky108]

Why would there be any liability to the utility if the fuse were on there side or the customers side? Unless you have people who put the wrong fuse back in, I just don't see it.
If the fuse were defective, they the liability should go to the fuse manufacturer. If the fuse were pre-damaged, then the liability should go to the customer.

I think we would do that if a customer asked. If you know someone on our board, we might do a lot more. But a transfer trip would be a NO.

 

[PowerDawg]

cranky108... it splitting hairs... we would be willing to own the fuses (and responsibility) that would blow if the customer got into something or we would own the breaker (and responsibility) that would trip if the customer got into something? I'm not able to distinguish between the two scenarios. The best thing for us (the utility) to do is to make the customer responsible for his own protection. What if the consultant sized the fuses wrong and we installed what he specified? I don't have the time to do the arc flash study for them and even if I did, that would be a clear case of liability. The best thing to do is not to say "we will do this option but not that option" but to simply say "we will not provide personnel protection for your employees... that is your job". When you discuss where the liability should go in a given situation, I recall that when someone gets seriously hurt that EVERYBODY gets sued... especially those with deep pockets... whether they had anything to do with the accident or not. Remember that some lady sued McDonald's over 'hot' coffee... and WON!

 

[jghrist]

You might get sued anyway if someone gets hurt. The claim might be that you were requested to provide something that would have reduced the arc hazard but refused to do so.

 

[PowerDawg]

jghrist... you are right, anyone can be sued for anything, anytime... its risk management. The way I see it is if we provided something specific for that chip mill and did not provide the same thing across our entire distribution system, we stand a better change of being sued because we did something 'out of the ordinary' or 'uncustomary' as compared to locations, stations or customers elsewhere on our system. When you do something special is when you are more likely to draw attention and get noticed. It would be hard to sue us (and win) if we provided the same service to the chip mill as we did everyone else. And again, I am hoping that most people (and juries) would agree that a reduction of an arc flash hazard lies with the company who employees the workers and not the utility.

 

[stevenal]

Any fuse on the primary side, sized to allow full load, temporary overload, and inrush; will not be effective in dealing with secondary side arc flash incident energy. The clearing time will be too long due to transformer impedance. Unless of course you use the high speed grounding switch I suggested above to increase the current seen by those fuses.

 

[jghrist]

Even with a main breaker, the customer is in somewhat of a dilemma if they need to work on the main breaker. Working on the main breaker, even unenergized by the utility, requires measuring the voltage to confirm that it is unenergized. If the incident energy is above 40 cal/cm² and the highest level of PPE available to the customer is 40 cal/cm², the measurements can't be made.

Would the utility be willing to confirm by measurement that there is no voltage on the high side of the transformer? If the transformer is the only possible source to the switchgear, this may be satisfactory.