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 Post subject: Switchgear Maintenace with energized BUS
PostPosted: Tue Nov 21, 2017 9:26 am 

Joined: Tue Nov 21, 2017 9:09 am
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What do you guys think of doing maintenance in the cell of a switchgear with a live bus. 4KV or 13.8KV with breaker removed, shutters locked closed, 125/250VDC blocking switches open. Asked to enter cubicle and inspect TOC switches and clean racking mechanism.


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 Post subject: Re: Switchgear Maintenace with energized BUS
PostPosted: Wed Nov 22, 2017 3:49 pm 
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colleyi wrote:
What do you guys think of doing maintenance in the cell of a switchgear with a live bus. 4KV or 13.8KV with breaker removed, shutters locked closed, 125/250VDC blocking switches open. Asked to enter cubicle and inspect TOC switches and clean racking mechanism.


Let's look at each potential hazard in turn.

First off, 4 kV or 13.8 kV...doesn't really matter. It is insulated and isolated behind either grounded steel panels or insulated panels. The hazard is identical to standing next to the cell with the doors closed.

The shutter statement is interesting...locked? Really, how? I've never seen that. The shutter rides on a cam follower mechanism on every breaker I've seen. Some of the very old Square D Model 3 stuff has it linked into the disconnect mechanism so you can definitely lock those shutters closed but most breaker cells just have a cam follower on one side that pushes on a bar that raises and/or lowers the shutters. You can open it by hand but not by accident...no way to inadvertently open the shutters. And what you are talking about doing is with qualified personnel, right? I could make a case for truly locking out the doors with unqualified personnel but not with qualified personnel. But then they wouldn't be qualified to clean it either so we're done before we get started. Either way this only potentially affects the 5/15 kV concern above so it's off the table.

I'm pretty sure that while you can use a tic (voltage tester) in the cell, it will go off because the shutters are insulated and not grounded, so they don't block EMI passing through the shutters.

Then we get to the secondary contact concerns. The statement is that there's basically no voltage here. Most contact blocks are guarded (recessed sockets) so even without removing power, it's dead unless work is going to involve cleaning and accessing this part. The description sounds like this is going to happen so...Dalziel's work shows that there is not the same kind of hazard from DC as AC. In terms of fibrillation, it's not going to happen so DC limits are higher than AC. Dalziel basically stated that it's more of a pain issue from the forced muscle contraction. It is generally accepted that 100 VDC is a minimum threshold and several public inputs were sent to NFPA 70E during the 2018 Code cycle stating this including plenty of references you can read for yourself. Setting the DC voltage limits the same as AC was basically a screwup. In terms of arc flash there is no evidence of significant arc flash at 130 VDC but there is definitely evidence for it at 250 VDC. So summing this up there is a small electrical hazard so it needs to be locked out. Your description did not mention testing for absence of voltage and obviously this would be required.


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 Post subject: Re: Switchgear Maintenace with energized BUS
PostPosted: Mon Nov 27, 2017 8:34 am 
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I sort of disagree with Paul. Being in a cubicle without a breaker and shutters closed is not the same as being outside the cubicle. I have been in cubicles and I have opened the shutters - it is fairly easy to do. You are now in a closed box, 6" from an energized bus, little protection. Why would I do that? Inspection of the insulator behind the cubicle wall (visual only).

As far as cleaning the mechanism, you are probably OK if you are qualified and you understand (been trained) in the hazards involved. The hazards include more than medium voltage stabs but the low voltage controls and CT wiring/terminals.


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 Post subject: Re: Switchgear Maintenace with energized BUS
PostPosted: Wed Nov 29, 2017 7:57 pm 
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I didn't say that it is not "easy" to open the shutter. I said that I've never seen one locked out except for the Square D Model 3, 4, etc., Isoflex medium voltage motor starter designs where it is interlocked into the disconnect handle, and second I said that INADVERTENT opening of the shutter is pretty unlikely. You can fairly easily open one...just push the shutter lever up. But its not a natural thing to do accidentally.

I've been in plenty of cells. And more than once I've jacked up the shutter to inspect the fiberglass, usually to figure out why the breaker is jamming up and won't go in right and doing this quite often reveals problems with the shutter or alignment or both.


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 Post subject: Re: Switchgear Maintenace with energized BUS
PostPosted: Fri Mar 22, 2019 7:12 am 

Joined: Thu Mar 21, 2019 3:12 am
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Do you have to be careful not to cause damage when jacking up the shutter or everything around it pretty sturdy?


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 Post subject: Re: Switchgear Maintenace with energized BUS
PostPosted: Fri Mar 22, 2019 4:20 pm 
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ClintSy wrote:
Do you have to be careful not to cause damage when jacking up the shutter or everything around it pretty sturdy?


Sturdy is hardly the word for it. Shutters are pretty much an afterthought by every manufacturer without exception. Most of them have better constructed name plates than shutters. They are required by some parts of ANSI standards so manufacturers install them but they are so poorly built by almost everyone that they are a primary cause of misalignment and mechanical issues with drawout gear and non-load break disconnects. That's important because misalignment is the primary cause of all kinds of dangerous conditions when it comes to draw out mechanisms. I can't say enough bad things about them and their use.They are all the way at the back of the cell. Half the time they either don't work, or don't close all the way, or are the problem in the first place. 95% of the time you have no reason to ever physically enter the cell. And if you do, you can take steps to insulate the bus, your tools, or yourself in the first place. If you don't, close the outer door and then the shutters don't even matter. I can't see any practical reason for their use that doesn't actually decrease safety with switchgear. Even as a contractor maintaining switchgear where they are so beneficial to my bottom line, I will gladly trade a few extra bucks in my pocket for safer equipment any day. And by safer I mean get rid of shutters. Over a century of crap designs has proven in my mind that they are adding needless complication to a fairly simple mechanism that fundamentally decreases reliability at a particularly critical point in the design from a safety point of view. I would much rather see this requirement stripped from the standards. There have been enough arc flash tests done that prove that although insulated bus may help prevent initiation of an arc flash, insulated bus does NOT prevent an arc flash from propagating once started, contrary to popular belief. So the shutter really doesn't do anything useful from a safety point of view except make someone feel safer when the door is open.

Physically shutters usually come in two varieties. The "external" kind are the easiest to deal with as far as manually opening them. You can visually see a piece of glastic that is either riveted or screwed to some kind of linkage mechanism. The glastic moves up and down on the linkage. The linkage is engaged by the breaker as it moves from the locked to the engaged position. This design is more common for switchgear as opposed to starters. The majority of these shutters open via a cam follower of some kind and close via gravity. Therein lies the problem...the shutter itself is usually wide and flimsy and kind of hangs down on the opposite side of the linkage. Or the linkage gets damaged in some way. Or some sort of contamination gets into the linkage and doesn't allow it to close via gravity. Or the drawings are written in say English but the builders speak say Farsi (GE, Siemens, probably others) and put the wrong screws in causing the screws to impinge on the shutter and jam it either partly open or partly closed, so it gets to the plant and then after the construction contractors leave someone like me has to go in and fix brand new equipment, hopefully before it blows up. Hopefully now that ABB, err I meant Hitachi owns them they will shut that plant down and fix the reliability problem but I'm not holding my breath.

In the "internal" design you will just see a flat glastic plate that is boxed in by either a round or rectangular fixed piece of insulated material. These have a sliding mechanism off to the side. They can slide vertically or horizontally. They are more common in the "starter" designs where for instance the fuse clips or the whole starter (Limitamp) or a disconnect (Square D or Allen Bradley) handle is connected to the shutters via some kind of linkage. Operating the handle causes the shutters to slide out of the way. Then the power conductors engage the bus via a cam that applies pressure and holds them up against the bus. Again contamination causes all kinds of issues but it is amplified by the fact that there is no way to get to the sliding surfaces to clean it. These ones tend to jam and when they do, they bust the glastic and/or metal linkages. There is more of a positive movement so self-imposed damage is more common.

The external type gravity-operated shutters are fairly easy to "force" open. It is so easy that it really isn't the right word for it. You just take an insulated tool and prop them open. It takes very little pressure to do this. You can usually do it from outside the enclosure. If you are forcing it at all either something is wrong or there is something keeping it from opening. With the others if there is a cam follower down the side (usually right hand) of the cell you can again just take an insulated tool and operate the shutter while situated outside the cell. If you have the type though that are positively operated, particularly medium voltage starters or low voltage MCC's this isn't going to work. It is often mechanically interlocked to the door. Inserting a screw driver in the right place to push a rod or slide back is all it takes to bypass the door interlock. Once you do this so that you can leave the door open, operating the disconnect handle or the breaker racking mechanism while carefully watching the shutter usually allows you to see how it works if it's not obvious. In cases other than the gravity close type, you may have to locate a nut or screw and disconnect the linkage to be able to operate the shutter with the breaker out, disconnect open, and/or door open.

You can look at the manuals for your equipment but chances are again...shutters are an afterthought so it's probably not in the manual. The best way to figure them out is by looking at the equipment and operating it so that you can see how all the linkages are timed.


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