Opening Large Bolted Pressure Switches

[infinity]

I was speaking to a buddy the other day and he commented that he had an arc flash problem while opening a 1600 amp, 480Y/277 volt Pringle style switch by pushing the big red open switch button. He insists that the load was shed but is being told that the switch must have had a load on it when the open button was pressed. Is it possible that this type of switch cannot be operated (opened) with any load connected? Here's an example of the switch type from Google:

 

[iwire]

In my area these are often remotely opened by a ground fault sensor so I have to assume they are designed to break a load.

 

[infinity]

I would think that in an emergency the switch would be capable of being opened without an arc fault incident. I wonder what the design standard says about these switches, does anyone know if they must be designed to be opened under load and not arc?

 

[Sahib]

I would think that in an emergency the switch would be capable of being opened without an arc fault incident. I wonder what the design standard says about these switches, does anyone know if they must be designed to be opened under load and not arc?

Any switch has a breaking capacity: if, during a fault condition or during motor start or similar condition, the switch is opened, it should able to break the current safely for which a breaking capacity in kA is given by the switch manufacturer. Perhaps in your case the switch breaking capacity was exceeded.

 

[jim dungar]

These types of switches require regular maintenance. Some statistics say there is a 50% failure rate for switches that have not been maintained (or at least operated) in the past 3 years.
The mechanical linkages often stick (due to cold flow of the grease?) casuing blades to not completely separate/open.

 

[infinity]

The switch in question is less than a year old. Someone shed the load, depressed the red switch open button and a huge arc flash ensued. Now he's being accused of not shedding the load, but can't a switch open under load?

 

[GoldDigger]

The switch in question is less than a year old. Someone shed the load, depressed the red switch open button and a huge arc flash ensued. Now he's being accused of not shedding the load, but can't a switch open under load?

If there were a transformer downstream, it would have been possible to shed the motor or other load but still have an inductive load on the switch.

 

[jim dungar]

The switch in question is less than a year old. Someone shed the load, depressed the red switch open button and a huge arc flash ensued. Now he's being accused of not shedding the load, but can't a switch open under load?

Yes, these are load break switches.

Did you have an 'arc flash', per 70E, or simply a 'flash' which was cleared by the arc chutes?

 

[Jraef]

If it is a Pringle brand switch, they ARE load-break rated.

I'm not sure, but I don't think there is the same kind of "arc chute" you may be used to in an enclosed circuit breaker. So at 1600A, there WILL be a big blue flash when you open this switch, even if there is very little load. For example of there is a large transformer being fed by this, the arc that happens when you open the switch may still be a very significant "flash" to the untrained eye, even though it is normal for the application.

The next question then is, how did he know? Was the door open when they opened this switch? Naughty naughty... Or is there a window in the switchgear that is using this switch? If so, was anything really damaged, or are they just shocked to see so much of the blue flash through the window? Sometimes people who are unfamiliar with the realities of switching something like a 1600A BPS are surprised by what they see and jump to conclusions.

 

[infinity]

This is a 1600 amp Pringle switch. Entire gear was enclosed (all covers were on) but when the button was depressed the 1600 amp switch opened and a large arc flash exited out of the ventilated cover at the bottom of the section. The 1600 was near the top, a 400 amp switch was in the middle and the vent section was below. The flash destroyed most of the 400 amp switch.

 

[GoldDigger]

This is a 1600 amp Pringle switch. Entire gear was enclosed (all covers were on) but when the button was depressed the 1600 amp switch opened and a large arc flash exited out of the ventilated cover at the bottom of the section. The 1600 was near the top, a 400 amp switch was in the middle and the vent section was below. The flash destroyed most of the 400 amp switch.

I would call that an arc flash. If the input was current protected to around 1600A, I would wonder whether there was a phase to phase clearance problem, missing parts, or an user-specced switch.

Possibly a broken part that caused a short?
Pringle will probably assist with a forensic analaysis of the switch.

Just what were the characteristics of the load which was shed, and how long between the shutoff of the load and the panic button push? Did the 400A switch contribute to it's own demise, or was it a passive victim???

 

[infinity]

Yes, there will be a forensic investigation but for now the poor guy who pushed the button is taking the heat. If one had to guess, during normal operation the connected load is less than 400 amps. As of now there is no indication of a mechanical failure or the evidence that something dislodged or was left within the switchboard and caused the damage.

I find it hard to believe that during the normal operation of the switch, even under load, would cause this to happen. Anyone know of the UL standard for these types of switches and as to whether or not they can safely be opened under load?

 

[jim dungar]

....The flash destroyed most of the 400 amp switch.

Definitely an 'arc flash'. A perfect example of why 70E is often interpreted to require PPE for routine switching, and why many people talk about a plasma event engulfing the entire switchboard.

Did any of the fuses blow?
Did the event start 'above' the fuses?

 

[GoldDigger]

Yes, there will be a forensic investigation but for now the poor guy who pushed the button is taking the heat. If one had to guess, during normal operation the connected load is less than 400 amps. As of now there is no indication of a mechanical failure or the evidence that something dislodged or was left within the switchboard and caused the damage.

I find it hard to believe that during the normal operation of the switch, even under load, would cause this to happen. Anyone know of the UL standard for these types of switches and as to whether or not they can safely be opened under load?

+1

To find out whether they can be safely opened under load, as well as any possible limits on reactive loads, check the model number with Pringle, not UL.

There is more to this situation than has so far met the eye.
Among other things, was this whole sequence triggered by a malfunction with possible side effects or was it a routine shutdown?

 

[infinity]

No fuses blew because it seemed to originate on the line side of the bus. This was a 4000 amp service switchboard with 5 service disconnects. The flash seemed to be somewhere in the back of the 400 amp switch. Fortunately for my buddy he was standing to the side of the section. The suit that he was wearing stopped just below his knees and the flash that came out of the grill was just above ground level.

+1

To find out whether they can be safely opened under load, as well as any possible limits on reactive loads, check the model number with Pringle, not UL.

There is more to this situation than has so far met the eye.
Among other things, was this whole sequence triggered by a malfunction with possible side effects or was it a routine shutdown?

Nope new building, new service, routine shutdown to do some work on another switchboard.

 

[GoldDigger]

No fuses blew because it seemed to originate on the line side of the bus. This was a 4000 amp service switchboard with 5 service disconnects. The flash seemed to be somewhere in the back of the 400 amp switch. Fortunately for my buddy he was standing to the side of the section. The suit that he was wearing stopped just below his knees and the flash that came out of the grill was just above ground level.




Nope new building, new service, routine shutdown to do some work on another switchboard.

So the input to the BPS was a tap directly on the 4000A service bus? In that case the potential fault current once an arc event started would be quite high. But I don't think that is necessarily relevant:

The flash seemed to be somewhere in the back of the 400 amp switch.

This makes me think that possibly the mechanical shock of the BPS opening caused an initial electrical failure in the vicinity of the 400A switch and its connection to the bus. That switch was probably still carrying load current at the time.

Opening a BPS can be quite a mechanical jolt even when there is no current flowing through it at all. I believe that one factor by which a BPS survives opening under load is by moving the contacts apart very quickly.

 

[Sahib]

There might be nothing wrong with the BPS except, perhaps, the way it was installed in the panel.

 

[infinity]

This makes me think that possibly the mechanical shock of the BPS opening caused an initial electrical failure in the vicinity of the 400A switch and its connection to the bus. That switch was probably still carrying load current at the time.

Yes the 400 amp was energized and had a load on it since it was unrelated to the shutdown. There is a barrier between the two switches so it seems strange that the arc could have rolled out from the 1600 amp into the 400 amp section.

 

[jim dungar]

Yes the 400 amp was energized and had a load on it since it was unrelated to the shutdown. There is a barrier between the two switches so it seems strange that the arc could have rolled out from the 1600 amp into the 400 amp section.

Shock protection barriers are not good at blocking plasma.
This is why many '70E experts' require Arc Flash Incident Energy calculations be made on the line side of the equipment, ignoring all internal protective devices unless specific compartmentalization has been employed.