Thermal overload devices

[MakingAComeback]

Recently at one of our sister plants an employee was burned when an arc occurred as he was inserting the fuses for a DC motor operated valve. What caused the arc is still under investigation.

However one of the persons who is involved with the investigation issued a proclamation which blamed the employee for putting his other hand in harm's way by depressing the reset button on the thermal overload device while he was inserting the fuses. This practice is widespread and has been followed for decades at our plant and evidently at other plants as well. The philosphy is that pushing the TO reset prevents an inadvertent start of the equipment while its power fuses are being inserted.

The investigator proclaimed that pushing the reset button was a totally wasted effort because the "49" contact on eutectic TOs does absolutely nothing when the reset is pushed unless there has been an actual overload event beforehand.

Well, well. We double checked the equivalent equipment at our plant and found that the "49" contact does indeed open when the reset button is pushed, so obviously the blanket statement that was issued is not quite on the mark. By the way it should be noted that the TO in question is vintage to put it kindly (once made by Cutler Hammer but no longer).

For those of you out there who have experience with the various thermal overloads that have been manufactured over the years how many of them do nothing at all when the reset is pushed, and how many of them behave like ours? Does it matter if the TO is eutectic or bimetal? Is there any consistency amongst the many makers of these things?

 

[jraef]

Wow, that concept is completely contrary to my experience!

An old fashioned thermal overload is nothing more than a spring loaded switch held closed by a ratchet mechanism. When the current heats the bimetal or eutectic alloy heater element enough, the ratchet (or pawl of the eutectic) releases and allows the spring to open the contacts, breaking the circuit. Once the heater element has cooled down, the mechanism is IMMEDIATELY available for reset, meaning that as soon as you push the reset button, the contacts will CLOSE, making the control circuit!

There is a concept that came into wide practice some 30+ years ago (I am not old enough to have seen them, but heard of this issue as a lad). That concept is referred to as TOL contacts being "trip-free". This means that when you push the reset button, and the heater has not yet cooled, the contacts will NOT close yet, so you cannot energize the contactor yet. Apparently before they came up with this, it was possible to defeat the TOL relay operation by having Joe-Bubba stand there with his finger on the reset so that the motor would continue to run. This of course meant that occasionally, the motor would catch on fire, but no biggie to Joe-Bubba I guess. Anyway, they eliminated that possibility by making TOL contacts "trip-free" a long time ago.

So your description of holding the reset button WHILE a fuse is inserted is absolutely asinine! That means that essentially, you are allowing the contactor to be closed BEFORE you insert the fuse, which essentially turns the fuse inserter into a contactor! No wonder he got burned.

The fact that for all these years, everyone has assumed that holding the button PREVENTED the contactor from operating is most likely a serious streak of lucky events in that when they tried it, it worked BECAUSE the TOL heater had not yet cooled! I'd say, off the top of my head, that this old practice in your plant is some kind of gross misinterpretation and possibly based on that lucky history; the guy who got burned was just the first unlucky one!

JRaef.com
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[jraef]

Forgot to finish my point...

So esentially I am in agreement with the person who implied that this practice of pressing the TOL reset while inserting the fuse is the cause of the flash burn. That practice is extremely dangerous!

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Hi jraef;
I have seen the type of overload action described by the OP. It was so long ago that I can no longer remember the details or manufacturer.
Almost all of the thermal overloads that I have seen and worked with do operate as you describe. It would seem that the unfortunate victim did not properly open the appropriate disconnect switch before inserting the fuses. This is safety a violation in many jurisdictions.
His second mistake was assuming that all thermal overload relays had the same action.
It has been my experience that the majority of accidents are the result of two or more issues.
I don't like to see anyone injured on the job. However, to deny that mistakes in procedure and understanding were made is to invite repeat accidents.
I would extend sympathies, but I would recommend that all workers remember to open the proper disconnect switches before working on circuits and the practice of using overload resets for safety devices is based on flawed reasoning and must cease.
respectfully

 

[jraef]

Excellent point Bill. A qualified electrician who could theoretically be working on a controller would (should) know better than to EVER assume a controlling device is providing protection. Anyone else should NEVER be allowed to open that cabinet in the first place.

The new Arc-Flash Safety rules should put an end to that, at least from a legal procedure standpoint.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

Good morning! Talking about thermal overload! The Stockholm Marathon of yesterday was one of the hottest - even African champion Bandawe collapsed. And it is getting hotter still today.

Well, that was a bit off topic.

I have a problem understanding what happened at that accident. Was TO downstream from fuses? Was the TO a thermal relay with a contact (the "49" contact) supposed to interrupt control voltage for a contactor coil or did the TO have main contacts that opened path for motor current? Is there any confusion regarding Reset and Off buttons?

I do not see much US electrical equipment and I am not familiar wit the exact terminology. That's why I ask.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Hi Gunnar;
It sounds as if the control scheme was calling for the motor to operate. The worker would have completed the power circuit by inserting the last fuse. When the fuse made contact the contactor would pull in. The worker was probably surprised by the closing contactor and inadvertently pulled back. At that point he would pull out an arc that was fed by the motor starting current.
Most of our overload relays are the same or similar to European equipment. Normally the contacts close when the reset is pushed. Years ago some were designed so that when the reset was pushed, the contacts would open until the reset was released. It seems that our victim worked in a plant where the old style overload relays were common, but mistakenly thought that a more modern unit had the same characteristics.
Respectfully

 

[rbulsara]

""The philosphy is that pushing the TO reset prevents an inadvertent start of the equipment while its power fuses are being inserted.""

What a oxymoron statment!!

I agree with jraef. Regardless of type or make of the TO, by definition a "reset" button will "RESET" the device bring it back to the "normal" operation. So why would a pressing a reset button would prevent a motor from restarting, when it is meant to allow the motor to start if it was tripped on overload earlier.

Please revise so called your standard practice!!!!!!! And would not bring this up in a litigation if you are the defender.

 

[rbulsara]

Only explantion I can think of is that if the TO relay has two separatge buttons one for reset and one for "Test". The "Test" button may prevent motor from starting.

But as stated earlier, the "right practice" is to open the disconncet upstream of the fuses before replacing them.

 

[MakingAComeback]

Folks,

I very much appreciate your responses.

Let me clarify a few things.

The person who was burned was an operator restoring the equipment to its in-service status after a clearance (tag out) was released.

There is no disconnect for this 250VDC motor operated valve; however, there is another set of fuses further upstream that supplies the Valve Control Center (our name for a DC MCC) in which this particular MOV resides.

The subject thermal overload removes power from the contactor's coil circuit as do all the other TOs in our plant.

Also let me assure you that pushing the reset button, the one and only button, does indeed OPEN the one and only NC overload contact when there has been no prior overload. This is a vintage Cutler Hammer TO with a part No. something like 10-3029-3 if memory serves me.

Furthermore, with the reset button pushed neither automatic circuitry nor an operator in the control room can close the contactor, and that is the sole reason the operator in the field pushes the button while inserting the fuses. That's how an arc is prevented as the fuses are being put in their clips.

Without having any further details at this time my speculation is that the flash occured because the fuse being installed somehow made simultaneous contact with the positive and negative fuse clips.

 

[rbulsara]

"There is no disconnect for this 250VDC motor operated valve; "!!!

How many other dangerous and unlawful (at least would be in most places) installations do you have?? More you detail practices at your site, more ridiculous and dangerous it appears.

Please have some professionals review your installation and operation & maingtenance procedures and correct the deficiencies.

 

[MakingAComeback]

With all due respect, sir, the circuitry we have been discussing in this thread is installed in a very large coal fired power generating station that was designed and built in 1970 by one of the country's largest two utilities.

Over the years I've been told that facilities like ours are exempt from the NEC if you are alluding to its provisions when you cite "other dangerous and unlawful isntallations."

Nonetheless, I would be most appreciative if you could steer me toward the sections in the NEC that would find fault (no pun intended) with the installation we are discussing.

 

[rbulsara]

M-A-C:

Exemption from NEC does not mean a license to make installations dangerous. In fact, facilities exempt from NEC are understood be under supervision of very qualified people, and your posts do not give me that impression.

Let me tell you, I am originally from a country where NEC is even not applicable, local codes were relatively loose, even there we did not have situations or practices that you mention.

For starters, since we are discussing motor circuits, refer to NEC article 430 in its entirety.

By your own admission there have been accidents, at least the one you mentioned, it could have been lot worse. So instead of trying to find obscure limitation in laws and regulations to justify a dangerous installation, fix your systems and practices.

Trust me even fully code compliant systems will have accidents and even those may not be defendable in courts, but at least one who complies with most common standards will not be considered negligent. Your last post is advertisement of negligence on your part.

 

[MakingAComeback]

"By your own admission there have been accidents, at least the one you mentioned, it could have been lot worse. So instead of trying to find obscure limitation in laws and regulations to justify a dangerous installation, fix your systems and practices."

I can't figure out how you came to this conclusion based on what I wrote. My interest is in finding justification for recommending a change if one is warranted.

At this point neither we nor you have enough specific information about the accident to lay blame on the "system and practices."

I read through section 430 of the NEC, but found nothing that fit our situation very well, but then again, reading and interpreting the NEC is not my forte.

Is it your position that fuses are not sufficient as a disconnecting means for DC motor operated valves?

By the way, this thread has steered way off course. I'm still very much interested in what kinds of thermal overload reset features folks out there have encountered.

 

[LionelHutz]

I think almost everyone posting here would take the position that pulling fuses from their holders is not sufficient as a disconnecting means.... unless the holders were specifically built to be used in this manner which would make it a fusible disconnect and not a fuse block.

The correct disconnect means is a properly applied and listed disconnect switch or breaker. In the USA the appropriate listing would be UL.

All of the overloads I have seen close the contacts when the reset button is pressed, assuming it is tripped. If it is not tripped the button does nothing.

 

[rbulsara]

Can't help someone in denial mode. You have much serious problems at hand that needs attention, you may have found out by accident here.

You can know as much as you want about OL relays, in the end relying OL device for lock out/safety is simply dangerous in my opinion and some others' who chimed in earlier.

And yes fuse by themselves are not cosidered disconncets. There must be a means to disconncet power to line side of the fuses for replacement.

NEC or not any device you want to work on must have a safe means of disconnceting power to it. Not sure why this is so difficult to understand.

Also onus of learing requirement of NEC or other applicable standards is on you. We can't solve all of you problems here nor anyone here has ever implied doing so.

 

[jraef]

MakingAComeback,

Without the previous acrimony, my opinion is that you have a situation in your factory that needs updating to bring it into line with current product design standards. That's not to say you need to change out your TOLs, just that you need to suggest a change to your procedures because they are based upon a unique quality of an outdated product that is now in direct conflict with standards in place everywhere else. The risk you run by not implementing the change is that any new equipment that comes in will NOT work the same as those old CH TOLs.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[MakingAComeback]

You, sir, are exactly right. My purpose in approaching this august group was to learn more about the kinds of equipment others are using for similar applications at their facilities, but obviiously I've done a horrible job of posing my questions.

I intend to find out if CH built this equipment to their own specifications 37 years ago or if they built it to my company's specifications. I'm reasonably certain that CH did not provide instructions to push the reset button while removing or inserting the power fuses, AND I'm most certainly NOT defending that practice; just telling it like it is.

Who knows how it got started? But we can probably assume that it has been handed down from one generation of operators to the next and the next and . . .

This incident may very well result in a corporate edict to change the circuitry to bring it in compliance with current industry practice.

Even if an edict from the company does not come down I intend to recommend interim and longer range changes to eliminate the potential hazard. Thanks for your insight.

 

[rbulsara]

M-A-C:

Thank you for understanding. You are always welcome here and I am sure you will be one of contributors on this site as well.

 

[waross]

Have you checked the OSHA regulations?
I am not in the US, but I was under the impression that OSHA frowned on changing fuses in a live circuit.
I don't think that your overload relays need to be changed. However I do think that serious consideration should be given to installing some type of disconnect switch so that fuses no longer have to be changed hot.
With a proper disconnecting means for the fuses, the overload relays will probably become a "non issue".
respectfully