controling conveyer belt

[OkiePC]

Forgive me for being a little bit anal here.
But thats the wrong medicine you are taking !
You experienced a local shortcircuit phase-to-phase, and your shortcircuit protection failed to contain it to the locality where it occurred.
Adding mechanical interlocks will not remedy that for the next time it happens. Mechanical interlocks can help but not guarantee against shortcircuits.

In stead, you should check your overload/shortcircuit protection system with regards to selectivity and shortcircuit breaking capacity.

We checked and had adequate local short circuit protection. I've seen a few other cases where, for unknown reasons, the fuses blow in a bus duct even though lower rated fuses in a local panel may or may not survive. Our plant already had a standard for reversing motor starter wiring, and after the incident I described it was reinforced.

Lancie's logic looks fine. If both start buttons (FWD & REV) are pressed at exactly the same time, the forward command will latch, and the reverse will be inhibited. I can't see any way to reverese directions without resetting the timer, which will provide the alarm and reversing delay required. I would recommend separate timers. What if the plant decides it wants a five second startup alarm and a 12 second reverse delay? Just my opinion...

 

[JesperMP]

the fuses blow in a bus duct even though lower rated fuses in a local panel may or may not survive

The selectivity of the overload/shortcircuit protection must insure that only the overload/shortcircuit protection closest to the overload/shortcircuit is tripped.
As you describe it, this criteria isnt met.

It is also possible that the problem is that the local overload/shortcircuit protection doesnt have sufficient current breaking capacity. This means that the short circuit current cannot be interrupted even though the protection is tripped. This then again means that the short circuit protection on the next higher level must/will trip.

If the local short circuit protection 'survived' (it can be reset, or the fuse can be replaced) then the problem was most likely the selectivity.
If the local short circuit protection 'was burnt' (there is visible damage from the heat) then the problem was most likely the short circuit current breaking capacity.

 

[rsdoran]

We checked and had adequate local short circuit protection. I've seen a few other cases where, for unknown reasons, the fuses blow in a bus duct even though lower rated fuses in a local panel may or may not survive. Our plant already had a standard for reversing motor starter wiring, and after the incident I described it was reinforced.

Everyone has seen this but seeing doesnt mean it should be that way. Fuses are not classified just by current rating alone, so if you have a situation where lower rated fuses may stay intact while larger main supply fuses blow then there is a problem.

As an example a motor ckt may have a time delay (slo-blo) fuse because of higher starting currents but the supply could be fast acting. This may work fine if the current stays within the rating of the main supply but with a short it could easily exceed the main rating and the main would blow first.

Whether its a Y-D start, forward/reverse or any form of motor control the use a plc should be applicable. For many years relays and timers were used to control the start/stop operation of motors and a plc is far more reliable then any electro-mechanical device.

For safety it does not hurt to be redundant in many situations. The use of mechanical interlocks on a starter can prevent problems in the case of a short circuit or binding contactor.

 

[leitmotif]

Bussman has a pretty good movie on foulups with fuses - quite the lihgt show. It definitely reinforced the idea that not only do you have to watch the current rating of the fuse but also the interrupt rating and the voltage rating.

Dan Bentler

 

[brucechase]

There is not adequate coordination of the protection devices. This is a good reason why everyone must be careful who builds/desings the panel. Every device in the panel must be evaluated as to its ability to properly interrupt the current and perform the protection without causing unnecessary loss of power. All fuse manufacturers, breaker manufactures, and other power protective equipment manufacturers supply curves just for this purpose. There are also a couple of good programs out there that does this (SKM Powertools). If you build a panel without doing the short circuit calculations and coordination, then you are opening up the end user to unexpected loss of production (as in this case) and possibly death (as in the case of too low an interrupting rating of the breaker. Just because someone can wire a panel doesn't mean it's safe.

 

[Lancie1]

Okie,

I have seen a few of those upstream fuses blowing, when the downstream should have went first. It is good to check the coordination to make sure it is correct. The next thing to check is for animal access. I have seen snakes, birds, squirrels, mice, rats, and bees cause short circuits in bus ducts and switchgear.

 

[OkiePC]

Our EE in that department checked the current ratings and interrupt current ratings of the fuses and circuit breakers involved and said there was no reason why the local fuses should have survived. We had IR pictures taken of the connections and found no problems. I just filed it away under the "unexplained" (with about a hundred other things that I've seen). It may be that the engineer did not do all the rigorous math that may be required and just told us it was okay to get us off his back. It may be that some other intangible force was at work. I understand current interrupt ratings and short circuit protection but I don't know what you mean by "co-ordination" and I have heard stories about succesive protective device causing conflicts but I don't have any good resources to learn about this. Can you guys point me to some info on "coordination" of protective devices?

 

[PhilipW]

Rockwell Notes on Coordinated Motor Protection

 

[Lancie1]

Okie,

You should know that just because the upstream devices have higher trip ratings and higher short-circuit currrent ratings, that does NOT mean that they cannot trip first. It depends on the time/current coordination curves.

"Electrical Device Coordination" refers to the art of matching the time/current curve of an upstream device with the downstream device. Each major circuit breaker or fuse will have a time versus current trip curve. The goal is to make sure that the downstream device trip curve falls to the left (shorter time, lower current) of the upstream device AT EVERY POINT. If the curves overlap at any point, then it will be possible for the upstream, or larger, breaker to trip first.

In the old days, these curves were printed out on translucent paper, and to check the device coordination, you overlaid the curves. Now many engineers use a software program, with the curves for various breakers and fuses contained in built-in look-up tables. These programs are very costly, so it is hard to justify buying one to check two devices. I have seen free stripped down "demo" programs (available from electrical distributors) that would check about 30 nodes. There are companies that specialize in this field and will come into your plant and do a complete study of all major protective devices. Basically they collect the nameplate data and cable sizes and lengths from your devices (and drawings) and then enter it into the program and hit "RUN".

If you only need to check two devices, then you can probably download these time/current curves from the fuse or breaker manufacturer. Bussman Fuses did have some good information on coordination, as well as coordination curves for all their fuses. The difficult part is in field-checking to identify the exact breaker or fuse model number.

PS. If your electrical supply is too small, here are a couple of ways to stretch it a little:

1. Check the voltage levels. It may be that you can change the main transformer tap to run at 2.5 percent higher voltage (The taps are usually in 2.5 percent increments). This will give you more power, provided that your equipment can stand slightly higher voltage.

2. If not already done, consider adding cooling fans to the main transformer. Many substation transformers have provisions for adding these fans. They can be retrofitted to increase the capacity.

 

[rsdoran]

Most fuse manufacturers have guides for what is called Selective Coordination:

What Is Selective Coordination?
It is not enough to select protective devices based solely on their ability to carry the system load current and interrupt the maximum fault current at their respective levels. A properly engineered system will allow ONLY the protective device nearest the fault to open, leaving the remainder of the system undisturbed and preserving continuity of service.

Exceprt from: http://www.bussmann.com/library/docs/spd02/SPDSection08.pdf

This is just one example, most fuse manufacturers provide similar. In some case they have online or downloadable software that may be used to properly select fuses.

Software examples:
http://www.littlefuse.com/cgi-bin/r.cgi/know_content.html?LFSESSION=K1wv1VUaMG&ContentID=32

http://www.bussmann.com/apen/software/

 

[brucechase]

Lancie1, Very good explanation of coordination. My biggest problem is that too many people just do something like put in a next higher size breaker above the fuse and say it is good and they don't understand how to "engineer" a system. And yes that statement includes engineers who think they know what they are doing when in fact they are unknowingly making a dangerous system. Coordination, arc fault, safety, (does the comma belong???) and interrupting capacity should be as much of the design as the type of plc.One slight correction to your option 1. Raising the voltage of the transformer does not increase the power, it just increases the voltage and the inductive devices will pull less current. The power a motor uses is the same (for the same load). Didn't mean to be too picky.

 

[RMA]

One of the best things about this forum is the amount of useful information you can pick up on the side, just by reading interesting Threads like this one. I never knew any of this stuff before and while I may not need it directly, I'm absolutely sure it'll come in useful some day.

 

[OkiePC]

Thanks for the education guys...I'm not an engineer and this is my first exposure to this subject. I am often asked to perform engineering tasks, so I'm sure this information will be invaluable in the future.

 

[Lancie1]

Bruce,

You are correct. I suppose I was thinking of those cases where I had a circuit breaker panel tripping out on high current. I raised the transformer voltage, and presto! everything runs okay. There is no more power then, but SOME power is better than ZERO power!