LOTO and fused disconnects

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moonshower

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Been dealing with this for decades and now its time to correct. 3 phase disconnects of various amperages are utilized for LOTO devices. numerous cycling throughout the day eventually destroy the mechanical mechanisms relating to the handle and its cheaper and cheaper design. We us Square D/Schneider electric.
I have searched the web for a high cycle repetition model made for many Off/On cycles and come up empty. I am almost at the point of modifying existing to include ball bearing pivot points in steel on steel on steel with a dab of grease as they are made. But that comes with added liability for sure.
Somebody somewhere must have the answer? Yes I am willing to change brands.
 
Disconnect switches designed to NEMA KS-1 and UL98 must be tested to have a minimum number of electrical operations under load, plus a higher number of mechanical operations. IIRC the mechanical operation value is 10,000, the electrical is I think 6,000 (but my memory is slipping on those details, I’ll look it up later). My point in bringing this up however is that most of the mfrs have a “General Duty” switch and a “Heavy Duty” version of the same switch. Both versions meet NEMA KS-1 and UL-98. The difference is that the GD versions just barely meet the criteria whereas the HD versions exceed them. People tend to buy the GD versions because they are cheaper, so double check if this is the case, because it might be as simple as making that change.

I used to work for Siemens, but they no longer pay me to care. Nevertheless as an Engineer, I found the Siemens “Vacu-Break” (now just called VB-II) to be probably the best design for AC applications (Sq. D is better for DC). The Sq. D and basically all others use an “energized hinge” design, going all the way back to the old knife switches on slate boards from the 1920s. The ITE (now Siemens) design uses a rotating arm with two blades that pivot into the contacts in two places, so there is no energized hinge and therefore no resistance across that hinge, which is the weakest link in all of those other designs. At one time Siemens had test data showing that the VB-II design FAR exceeded the NEMA & UL requirements, outlasting all energized hinge designs by 3:1. Again, I’m not selling them, they haven’t sent me a paycheck in over 15 years. I’m just relating what I learned at that time.
 
Jraef said:
Disconnect switches designed to NEMA KS-1 and UL98 must be tested to have a minimum number of electrical operations under load, plus a higher number of mechanical operations. IIRC the mechanical operation value is 10,000, the electrical is I think 6,000 (but my memory is slipping on those details, I’ll look it up later). My point in bringing this up however is that most of the mfrs have a “General Duty” switch and a “Heavy Duty” version of the same switch. Both versions meet NEMA KS-1 and UL-98. The difference is that the GD versions just barely meet the criteria whereas the HD versions exceed them. People tend to buy the GD versions because they are cheaper, so double check if this is the case, because it might be as simple as making that change.

I used to work for Siemens, but they no longer pay me to care. Nevertheless as an Engineer, I found the Siemens “Vacu-Break” (now just called VB-II) to be probably the best design for AC applications (Sq. D is better for DC). The Sq. D and basically all others use an “energized hinge” design, going all the way back to the old knife switches on slate boards from the 1920s. The ITE (now Siemens) design uses a rotating arm with two blades that pivot into the contacts in two places, so there is no energized hinge and therefore no resistance across that hinge, which is the weakest link in all of those other designs. At one time Siemens had test data showing that the VB-II design FAR exceeded the NEMA & UL requirements, outlasting all energized hinge designs by 3:1. Again, I’m not selling them, they haven’t sent me a paycheck in over 15 years. I’m just relating what I learned at that time.
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Great factual data. Only use HD Square D. Appx. 3600 throws a year by production employees. In a hurry tolerated by the administration at the facility. Haven’t seen one electrical failure. But mechanical for sure. Will check out the Siemans product. “payed to care”. That’s a classic.
 
why are you locking out a disconnect that many times? it is only really needed if you are doing electrical work and might get electrocuted. if it is just to do some kind of routine operation, there are other steps you can take that are acceptable.
 
Have you looked at a cam style rotary switch, if this is a low ampacity application that doesn't need visible break, and tolerate the fuse holder being separate from the switch.
 
All thoughts. To answer a question . LOTO is a machine setup requirement in industrial settings. Designed to protect employees from machine operating while preforming setups.
 
Ok so assuming OSHA…

There are several different LOTO Codes. Electrical maintenance for instance falls under 1910.269 (4 LOTO procedures for utilities) or subchapter S. General maintenance falls under Subchapter J. Operations falls under subchapter O. Reason I’m saying this is they are probably using the wrong procedures. 3600 times per day means LOTO every 24 seconds. This is an application that screams for a safety system using gates, pressure pads, area scanners, light curtains, etc. That way production can do whatever it is that they are doing without ever bothering to lock anything out. You are probably looking at a SIL 2 or 3 system and it will be vastly safer than what you have now because it won’t rely on someone remembering to lock out.

Make sure you have someone that knows what they are doing develop the rating at least the first few times. Do NOT let your safety department do it. SIL 1 is standard controls. SIL 2 requires “safety” grade equipment. SIL 3 requires full redundancy. SIL 4 is triple redundant. Often by designing properly (process design) or slightly modifying existing you can keep it at SIL 1 or 2. For example a local plant had SIL 3 E -Stops. By definition an operator can’t exceed SIL 1. Another had a complicated SIL 2 system for a catastrophic fan blade failure based on essentially an incredible event (not a credible one). With proper analysis the entire crazy system that was proven to not even work (fan blades failed but detection never triggered) was removed and replaced with routine vibration PMs.

Note that often you have to retrain your safety morons who never learned how to read. They see subchapter J as the one and only LOTO procedure completely ignoring everything else. Plenty of plants routinely have operators entering production cells safely without LOTO. This does NOT apply to maintenance but it does for production .

Second issue which I see commonly is operators think they can/should open and close disconnects under load. This is a really bad practice. Disconnects are designed to do this a few hundred times for disconnect ONLY. They are called disconnects and not switches for a reason. Ask your distributor what the closing under load rating is and watch them squirm. Contractors are rated for tens of thousands of cycles. If you cycle a lot you can use a soft start or a drive to increase this even more. When disconnects fail they explode quite often. The best solution is see above…stop doing this But if you can’t fix stupid you can at least remove the electrical load. You can install aux contacts in the disconnects that are “early break”. That way you can de-energize the load (trip the contactor or drive) before the actual disconnect opens. Preferably you also trip out the E-Stop system forcing a reset and encouraging them to use the controls for what they are meant to do. Orderly shutdown THEN open disconnect.
 
i realize you didn't start this to discuss your process but i am curious why so many production changes, this doesn't sound very efficient.
why not replace disconnects at six months, if they are wearing out you got your money out of them.Plus knowing they are in bad shape and not changing them is a liability. Maybe the added expense will get mgt.
involved in the process. good luck.
 
wwhitney said:
But the OP said 3600 times per year. So with a 250 day work year, that's ~14 times a work day.

Cheers, Wayne
Click to expand...

That’s better?

I just checked. According to UL 98 it must survive 6 operations at 200% of rated load, 6,000 at rated load, and 4,000 no load, at 6 per minute. So at 3600 operations per year under load anything over 20 months is living on borrowed time. Schneider claims their heavy duty switches exceed UL 98 by 300% but they don’t say what spec the 300% refers to.

If you did it not under load (using the auxiliary contact) it increases the rating to 10,000 operations following UL 98. Don’t forget IEC AC3 duty is typically over a million open/close operations. That extends life to 33 months. VFDs with a “safe torque off” function are rated for 100,000 hours or 11 years. So for operations use you can use a simple key switch or something similar tied to safe torque off and increase things out to over 10 years.
 
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