Opening a 15kV switch on a 2500kVA 480V for lockout SAFETY HAZARDs? 1

[bdn2004]

I'm working at this plant that locks out the 15kV outdoor fused disconnect switch at a 2500 kVA unit sub whenever they work on any of the 480V motor control centers it feeds.

They were forced to do this after discovering the arc flash energy levels were too high at the mains of the MCCs.

The 15kV fused switch failed recently. We're trying to justify our remedies by claiming this is a safety issue operating the 15kV switch for this purpose. I'm struggling to say why...

Any thoughts?

 

[rbulsara]

Sounds like an overreaction to arc flash fobia. One should be able to "trip" an LV breaker from a safe distance. Add a shunt trip and trip from remote, if you have to.

How did the 15 kV switch fail? Was it a load break or no load break switch?

Typically HV switch is opened under no load, that is only after opening the LV main. Even if the the HV switch is load break rated, they have limited number of mechanical operations over its life. Far less than a LV breaker would have.


Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

Agree with Rbulsara's points. What type of switch is it? Your plant may have created a bigger hazard (risk) than it had to start with.

There are wireless remote operators available for ever type of CB out there, no mods needed to your switchgear. Just magnetically attach, walk outside the AFB, push the button.

 

[rbulsara]

I should add that the LV main should be changed to electrical operated one, for remote closing and opening, if you cannot safely open or close from a safe distance.

Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

There is probably a very high arc flash energy at the line side of the main 480 V breaker if it is fed directly from the transformer. This is because of the long interrupting times for a fault on the line side where clearing is by the 15 kV overcurrent protection. This would justify opening the 15 kV switch if you had to work on the main breaker, but beyond the main breaker, you should be able to de-energize by tripping and locking out the main.

 

[bdn2004]

Thanks for the good answers.

But I'm not really looking for remedies as many have suggested. I'm was struggling to say WHY this is a safety issue. I'm involved in the funding of these projects. Not the design. And a big safety flag, gets the attention of management.

As for a LV breaker...there's not one. Either at the MCC's or on the secondary main. Everything is fused switches. This stuff is probably 30 years old. They claim the 15kV switch failed from overuse, as it wasn't intended to be opened every other week like its being used now. And no its not a load break switch. I don't think its opened under load.

And jghrist, the MCC feeders are fed directly off the secondary main fused switch. One of the suggestions from the arc flash study was to put a separate isolating fused switch on these feeders, and fuse them down and faster.

Any more thoughts...?

 

[Zogzog]

No mina huh? Paper mill by chance?

This IEEE white paper should help

 

[stevenal]

At 15 kV you have less available current and shorter clearing times to reduce the arc flash incident energy.
If you are not dropping load, then you are dropping the transformer magnetizing current which can be worse. Suggest getting a switch rated for this duty.

 

[bdn2004]

Thanks for the advice everyone.

I sketched up the basic system and its attached. The proposed at the bottom is what the engineering firm who did the arc flash study suggested. Keep in mind, that this operated event free for 30+ years.

All comments are welcome.

 

[dpc]

Main 480 V breakers are generally about the worst arc-flash situation in a plant. Faults in the main breaker section have to be cleared by the upstream fuse (or breaker) and clearing times are long.

Since you don't have a main breaker, then all of your feeder breakers are basically main breakers as far as arc-flash is concerned.



David Castor

http://www.cvoes.com

 

[Zogzog]

Take a look at these remote switch operators, they are available for any type of switch or breaker and would be a more cost effective and simple solution.

http://www.remoterackingsolutions.com/remote-switch-operators.htm

 

[Elmir]

Although the equipment has operated 30+ years event free, the condition of the equipment is not as it was new. The condition of the equipment is dependant on the maintenance history, environment and number of operations.

We perform routine maintenance on the sort of equipment you describe. Our policy is to wear PPE which includes a flash suit. We have experienced switches fail during maintenance shutdowns and during switching events on 15 and 25kV systems. The causes are all different, new and old switches, different manufactures.

 

[jghrist]

Faults on the 480 V bus in Substation Z should be cleared by the transformer secondary fuse. This is not as bad of a situation for arc hazard as situations where the primary fuse has to provide 480 V fault clearing. You may still have high arc energy because the fuse has to coordinate with the downstream fuses so cannot act very fast.

The proposed solution of adding a 1600A fused switch will not help the problem at the main switchgear, but does provide a place to de-energize the MCC without opening the 15 kV switch. You could use a remote operator to open the new 480 V 1600A switch. You will still need to have PPE for the arc energy at the MCC in order to verify that the MCC is de-energized with a voltage detector. The 1600A fuse will still have to coordinate with the MCC 450A fuses, so there may not be a reduction in arc energy at the MCC.

A better solution would be to use a breaker to feed the MCC. You could then use a maintenance switch that changes protection to an instantaneous trip whenever you need to maintain the MCC.

 

[bdn2004]

Jghrist,

The proposed solution is one large 1600A fused switch. I wondered why they didn't suggest 3 separate switches, say all fused at 800A. They are telling us that even with the 1600A switch we are looking at a PPE level 3, not preferred but better than we have now which is causing the issues. Is this solution worth persuing?

As far as these remote actuators go..
I'm guessing you take the lever off, and permanently install the actuator over the lever cam? I'll research but thought you may know off the top of your head.

 

[Zogzog]

You do not need to permantly mount anything, or remove anything. Give them a call with your equipment type and they will send you details and will do a demo at your facility on your equipment.