Thanks. I was unfamiliar with the terms you were using. I assume this has nothing to do with the regular start/reset/stop buttons that pickoffs use to stop/start the belt.
That doesn't happen here, as far as I'm aware.
"On Topic" Operations Management Jogging Knife Switches
- Thread starter Integrity
- Start date
Thanks. I was unfamiliar with the terms you were using. I assume this has nothing to do with the regular start/reset/stop buttons that pickoffs use to stop/start the belt.
That doesn't happen here, as far as I'm aware.
Integrity
Binge Poster
You are welcome.
You are correct.
It is the switches that are near each motor that this potential hazard exists.
Management isn't supposed to use the disconnects as a means of starting/stopping/jogging a belt, but the reason is mainly the protection of equipment and not for safety reasons.
For the most part, it shouldn't be possible to jog one section of a belt by using a disconnect. UPS design calls for each belt to be interlocked to the belt that it feeds. This is done with current sensors (pre '88ish) or SSIs. Obviously, a lot of this depends on the age of the building and what the design called for at the time and if any changes were made over the years. And there are also a few exceptions to the interlock requirement.
Again, this is done to protect packages and equipment. If a belt is shut off and the belt feeding it is still running, packages are likely backing up/jamming with the potential for damages and packages setting on a transition point between two belts provides an opportunity for a bag or package to be sucked in and cause more damages or ripping a belt.
The video posted can happen, although it is unlikely and the disconnects should have an arc flash shield inside to prevent that from happening. The physical "knife blades" aren't typically designed for constant switching and will prematurely wear out if this is done regularly. Also, there are combo motor starter/disconnects that have a motor starter in the disconnect, although I don't think they are used often at UPS, at least in larger facilities.
For the most part, it shouldn't be possible to jog one section of a belt by using a disconnect. UPS design calls for each belt to be interlocked to the belt that it feeds. This is done with current sensors (pre '88ish) or SSIs. Obviously, a lot of this depends on the age of the building and what the design called for at the time and if any changes were made over the years. And there are also a few exceptions to the interlock requirement.
Again, this is done to protect packages and equipment. If a belt is shut off and the belt feeding it is still running, packages are likely backing up/jamming with the potential for damages and packages setting on a transition point between two belts provides an opportunity for a bag or package to be sucked in and cause more damages or ripping a belt.
The video posted can happen, although it is unlikely and the disconnects should have an arc flash shield inside to prevent that from happening. The physical "knife blades" aren't typically designed for constant switching and will prematurely wear out if this is done regularly. Also, there are combo motor starter/disconnects that have a motor starter in the disconnect, although I don't think they are used often at UPS, at least in larger facilities.
1
10 Pt
Guest
Great overview. Thx.
On the older systems, which would short out first:
1. The relay safety disconnect (or local start/stop box) or:
2. The motor starts?
Can the disconnect (start/stop) be locked out? To me it looks like that's a management privilege (if at all) or an action provided for a designated and approved service technician.
I don't believe it's jurisdicted by the CBA unless it puts employees in an unsafe work environment but it may set a bad (conflicting) example.
Integrity
Binge Poster
True they are not.
The main reason is for safety of personnel and not mainly for the protection of equipment.
Integrity
Binge Poster
Seen some of these but many conveyors don't have disconnect interlock circuits and management can still incorrectly shut live disconnects off even if interlocks are installed.
Integrity
Binge Poster
Not done primarily to protect packages and equipment but to protect personnel.
Integrity
Binge Poster
I would imagine the potential for explosion would depend on the point of failure.
1
10 Pt
Guest
Rarely do motors explode unless there's a high leg dead short in the three phase supply. I didn't watch the video but motor explosion (if that was what it was) is rare.
Integrity
Binge Poster
It was not a motor. It was a safety switch/disconnect.
1
10 Pt
Guest
Over used.
1
10 Pt
Guest
I have now watched the video. That was a serious dead short and I have never seen a safety disconnect box explode like that except when some technician was using a plasma cutter too close to the supply terminals. That was very similar.
If the wire lugs get loose over time I can possibly see a dead short issue but typically the motor would fail (if three phase) because a three phase motor will not start with only two hot legs supplying it It'll typically burn (melt) the terminals first and then maybe short across to ground.
That clip wasn't the norm.
wkmac
Well-Known Member
This does not happen in my facility as a result of how the line power to the conveyor is configured. Our line power is wired from the MCP into the safety disconnect and from there into a VFD and finally into the conveyor drive motor. Throwing the safety disconnect cuts power to the VFD which reads the power loss as a fault and thus it shuts off the entire conveyor train through a low voltage signal to the conveyor train interlock. One has to then clear the fault at the VFD, in this case position the disconnect in the "On" position and restart the conveyor. "Jogging the Safety Switch" which is not only against UPS methods but also NEMA safety standards is not an option in our building and doing so would actually be counter productive.
Our solution in a package volume overload condition is to flow the volume to another conveyor if possible until volume on the overloaded belt normalizes or we go as far as cut off the unload and then bump the entire conveyor train from the control console until problem is corrected and then restart the building. This incentivises the sort operations to not create an overload condition to begin.
watdaflock?
Well-Known Member
Please define jogging. Is jogging a union job? If not, don't worry about it. If it really is dangerous, good that management is doing it and not union employees.
Hope this helps.
Integrity
Binge Poster
How old is the building?
Integrity
Binge Poster
Jogging a conveyor general means running it for short intervals.
watdaflock?
Well-Known Member
Why is running the belt for short intervals dangerous? It may be more dangerous running it non stop, if it causes egress.
wkmac
Well-Known Member
20 years.
UPS Preloader
Well-Known Member
We had a preloader doing that in my building at one time. If management is doing it, my guess is they are doing it to save time. When you power the belt back on using the knife switch, the belt starts immediately without any delay or warning. At least that's how it works in the building I'm. Doing it this way, is definitely a safety hazard.
Knowing that you'll probably ask me how I would handle the situation, I'll save you the trouble by letting you know that I would confront the supervisor and request that this action cease immediately.
Integrity
Binge Poster
Thanks for the participation.
No, I wasn't going to ask that but thank you.
Excellent post!
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