Load break switches and MV motor control

[tmahan]

We are presently looking at two different designs for 5kV class, 720A motor controllers.

Design one's vendor is quick to point out that their design incorporates a Load/break fault-make switch (800A rated breaking) for isolation.

Vendor number two is quick to point out that their design does not use the isolation switch to make or break any current (.4 amp rated). Instead all interruption is handled by the main contactor. Investigation into contactor interuption capability shows a rating of 7200A.

Right now, I think I prefer the use of the contactor for interruption as opposed to the use of a switch, but have limited experience in this area.

Does anyone have strong, well though-out, opinions on this subject? Or is this a case where we should just let our purchasing dept decide

 

[busbar]

By no means comprehensive answer, but contactors having ready visible-contact drawout/isolation capability should rate highly. Procurement dealings are essentially one shot—switchgear features nag you for decades.

 

[jbartos]

Suggestion: A more accurate answer would need more information about the application, namely,
1. The switch advantage is in its retentive memory than means, if there is a power failure the switch will be closed and wait for the power to be restored while you may enjoy nice time on vacation. The contactor normally does not have this feature since its memory is nonretentive. Someone has to start the contactor, e.g. via a push button. This may require to come back quickly from your vacation to have the load on.
2. The switches tend to be less expensive.
3. The switches tend to be more reliable over the long period of time.
4. Ultimately, the switch is somewhat clearer to operate in many application settings.
5. Etc.

 

[tmahan]

Jbartos
Clarification:
The load break switch is not being used in replacement of a contactor. The contactor will be present in the starter regardless.... My question to those of you who have more experience than I using, designing, supporting and maintaining medium voltage controllers is "Is it better to have an isolating switch capable of interrupting the load current or should interruption always be done with the contactor since it has a higher interrupt rating than the switch?"

 

[jraef]

Is the main contactor going to be a vacuum contactor? If not, a 5kV air break contactor itself will be several times larger, possibly even larger than the Load-Break switch. In addition you may still need to add the isolation contactor for lock-out purposes.

So assuming that you will be using a vacuum contactor, the best argument for load-break rated switches comes from the concept of "what if" engineering. "What if" the vacuum bottles leak or the contacts weld? The one drawback of vacuum contactors is that they will NOT interrupt a load if the vacuum is lost. The gap between contact surfaces is only about 1/8 inch. So "what if" you lost vacuum or contacts welded in 2 phases while the motor was running? If someone grabs that isolation switch handle to try to stop the single phasing motor, the switch may literally explode in front of them. Sure it is a lot of "what ifs", and vacuum contactor manufacturers will be quick to point to the safety record of their devices, but for the difference in cost I feel it is a prudent investment in product safety. I have had to pull that switch under load once when the contacts welded after a lightning strike. Even with the Load-Break switch it was a memorable event, but I lived to talk about it. That which does not kill me, makes me stronger... and pissed off!

 

[elecman]

I use a lot of contactors for motor control and without exception use a simple fused contactor arrangement. The switch in addition to the contactor seems a waste of money and I can't see any material benefit. Design one's vendor is bullshitting.
In Europe there are probably no air break contactors in production, certainly not among the leading manufacturers, and I should say either vacuum or SF6 would be equally as good as each other. The fuse will give short circuit protection whilst the contactor gives all the other functions when used in conjunction with an intelligent multifunction relay with logging capabilities, eg Motor Manager or GemStart.
I'd expect a Vacuum or SF6 contactor to do much better than break 7.2kA, by the way. Most vacuum contactors are usually OK on the very rare occasions when you do lose vacuum - check the manufacturer's test certificates. If he doesn't have any independently witnessed test data forget about him.
Don't ever leave it to the purchasing people - you'll end up with the wrong thing.

 

[redtrumpet]

elecman - why do you say vendor No. 1 is full of BS? On an MV circuit you require a lockable, visible break point for motor circuit isolation. North American manufacturers prefer using an isolating switch on the line side of a fixed contactor. I believe IEC manufacturers prefer using a withdrawable contactor that can be locked in the withdrawn position. The former design is predominant in North America. I have seen a Siemens MCC that used the withdrawable contactor. I suppose if you never saw an Allen-Bradley or Square D MCC with isolating switches for each starter, you might believe vendor No. 1 was full of crap. What can I say? We are fond of our visible break switches here in Canada and the US. I personally know a colleague who rejected a bid for the withdrawable contactor type because he wanted the isolating switch. I guess you go with what you know.

 

[jbartos]

Suggestion to tmahan (Electrical) Jul 8, 2002 marked ///\\Jbartos
Clarification:
///Thank you for the clarification.\\The load break switch is not being used in replacement of a contactor. The contactor will be present in the starter regardless....
///This was not so apparent in your first posting. Therefore, I presented generic differences between contactors and switches.\\ My question to those of you who have more experience than I using, designing, supporting and maintaining medium voltage controllers is "Is it better to have an isolating switch capable of interrupting the load current or should interruption always be done with the contactor since it has a higher interrupt rating than the switch?"
///The interruptions under normal operating conditions shall be performed by contactor (obviously with the generic drawback of contactor having nonretentive memory, i.e. need to be reset/restarted after power outages in most cases). The isolation switch capable of interrupting the load current is there for safety reasons, e.g. emergency (since there may be expensive values at steak, e.g. the motor, controller, process, health, etc.\\

 

[vanskat]

Just a few comments:

Many installation standards require that MV circuits be installed with lockable, visible breakpoint for motor isolation as reiterated by redtrumpet// This facilitates physical lockout/tagout exercises for maintenance and may prevent certain incidents.//

Non retentive memory may not necessarily be a drawback, as there are certain occasions when an electrician is required to physically check the circuit for deficiencies, depending on the application etc.. (That's one of the main reasons why I do not install automatic reset overloads)

Finally, if I had interpretted the previous posts correctly, it may be possible to get the best of both worlds. On the market are some "bolted pressure switches" that offer the load break capability (equipped with arc tips and arc chutes)

 

[brupp]

I believe there has been confusion on this issue from the original post. As redtrumpet explains, the two options are a visible disconnect switch or a drawout contactor. Some manufacturers do one and some do the other. I don't believe anyone provides both. In either case, the disconnect switch should never be used to break the load. If the DS has a load break rating, great. When it goes to break the small charging current or CPT current, it will do so with only the smallest of arcs. I would never consider using that switch to break the load however. I would instead trip the entire bus in the case of a contactor failing to open the load circuit, then disconnect the load by opening the DS or withdrawing the contactor. If you ask any operator I have to think he or she will agree.

 

[redtrumpet]

Thank you, brupp. I also thought there had been much confusion on what is normally a straightforward issue. I specifically used the phrase "isolating switch" and ignored whether there was a load break capability. I am in complete agreement of tripping the upstream breaker to the bus in the event of a failed contactor.

 

[infomaniac]

I did not know we were looking at our own design? lol

P.

 

[cuky2000]

Consider taking advantage of the Contactor-Fuse Coordination to provide higher interrupting rating of the device. For fault above 7200 A the fuse will operate before the contactor and vice versa for fault bellow the contactor interrupting rating.

Under this condition the Contactor-Fuse assembly is typically rate for 50 kA. This practice is technical sound in compliance with NEC and other electrical standards.

There are large application experiences that support this method of rating the contactor-fuse assembly for higher interrupting capability. Similar analogy could be experience on the insulated cable protection for SC with the same fuse.

I suggest to consider to plot a protection coordination curve using standard electrical software or by hand if you do not have one available.

To illustrate this issue with other sources please see the enclose reference on page 11-8

http://www.ch.cutler-hammer.com/unsecure/cms1/I1.PDF

I hope that could help your specific application.

 

[cuky2000]

Consider taking advantage of the Contactor-Fuse Coordination to provide higher interrupting rating of the device. For fault above 7200 A the fuse will operate before the contactor and vice versa for fault bellow the contactor interrupting rating.

Under this condition the Contactor-Fuse assembly is typically rate for 50 kA. This practice is technical sound in compliance with NEC and other electrical standards.

There are large application experiences that support this method of rating the contactor-fuse assembly for higher interrupting capability. Similar analogy could be experience on the insulated cable protection for SC with the same fuse.

I suggest to consider to plot a protection coordination curve using standard electrical software or by hand if you do not have one available.

To illustrate this issue with other sources please sees enclose reference on page 11-8 and technical data on pages 11-17 & 11-20

http://www.ch.cutler-hammer.com/unsecure/cms1/I1.PDF

I hope that could help your specific application.

 

[redtrumpet]

cuky2000 raises a good point, as usual. tmahan, you'd better figure out what your concerns are before the vendors eat you for lunch, or else farm this out to someone who knows what they are doing.

Briefly, though, in case you haven't figured it out yet: the contactor should perform all motor switching duty and interruption of overloads. The fuse must be coordinated with the interrupting capacity of the contactor and the thermal overload curve to ensure the fuse blows first for short-circuit currents above the contactor interrupting capacity.

The load-break switch is a red herring; in practice you will presumably only use this for isolation purposes, and depending on your safety and maintenance procedures and your own opinion, the withdrawable contactor design without a switch may also prove completely adequate.

I think you are confusing two separate issues when you compare the load-break capability of the isolating switch with the interrupting capacity of the contactor. The two are not related.

 

[jraef]

Redtrumpet, I respectfully disagree.
//The load-break switch is a red herring; in practice you will presumably only use this for isolation purposes, and depending on your safety and maintenance procedures and your own opinion, the withdrawable contactor design without a switch may also prove completely adequate.\
Again, I reiterate. What if the vacuum bottles fail? How is it going to interrupt any load? The fact that it is withdrawable (sic) means nothing if you cannot disconnect the power first.

//I think you are confusing two separate issues when you compare the load-break capability of the isolating switch with the interrupting capacity of the contactor. The two are not related.\
I do agree that the system should be designed such that under normal circumstances the load is interrupted by the vacuum contactor. But since, as you pointed out, we in N. America like having visible blade disconnects anyway, why not spend a few extra bucks for the load-break capability? It just seems to me to be prudent insurance.

"If it were not for electricity, we'd all be watching TV in the dark!" ...George Goebell.

 

[redtrumpet]

jraef:
"Again, I reiterate. What if the vacuum bottles fail? How is it going to interrupt any load? The fact that it is withdrawable (sic) means nothing if you cannot disconnect the power first."

What if an electrical component anywhere in the entire power system fails? Based on your logic we would need load-break switches ad infinitum, load-break switches backing up every circuit breaker and contactor in the plant. Are you in a facility where vacuum bottles are failing every day? There are other ways of disconnecting the power. As brupp pointed out, trip the upstream feeder breaker to the starter or the MCC bus if a contactor fails. Remember, all those switches need to be maintained and exercised and are one more component that can fail.

There is nothing wrong with the isolating switch having a load-break rating. You can even open the switch when a contactor fails if you like (make sure you stand well to one side - you just never know). I just don't think it's a big enough factor to bias the purchasing decision against the withdrawable contactor style of starter.

 

[jbartos]

Suggestion to redtrumpet (Electrical) Jul 12, 2002 marked ///\\The fuse must be coordinated with the interrupting capacity of the contactor and the thermal overload curve to ensure the fuse blows first for short-circuit currents above
///Definitely true. However, it is better to have the fuse opening the circuit somewhat below the contactor interrupting capacity. A good design margin could be somewhere around 10 to 20 percent since there may be contributions from other branch circuit motors. These contributions may vary, e.g. someone installs higher HP motor or motor with higher locked-rotor currents and the short circuit current contributions will be bigger. They will elevate the short-circuit current above the value calculated in the past. If there is no margin left, the contactor could severely be damaged by the fault exceeding its interrupting ratings due to the higher rated fuse.\\ the contactor interrupting capacity.

 

[jraef]

redtrumpet:
OK, you made your point, it's valid and I respect it.
I would never go so far as to recommend load-break switches everywhere but, as opposed to isolation-only switches, the cost differential is justified in my humble opinion. As I mentioned, I once had to grab and pull one of those switches and I am thankful that it was rated to break that load. This will probably color my opinion for the rest of my life and no reasonable amount of cost saving is going to change my mind. "If it were not for electricity, we'd all be watching TV in the dark!" ...George Goebell.

 

[jbartos]

Suggestion. Please notice that a no-load isolation switch is often installed in power distribution systems for isolation only. It is operated when the power distribution line is deenergized. It may have a special lock to be unlocked before the switch is operated. The safety switches have, in addition to the isolation feature, an emergency interruption under full load. Supposing that you will have a trainee at a machine and you do not like some operation, you just open this safety switch or if there happen to be an emergency push button, then you stop the machine by the emergency push-button.