Power Circuit Breaker Trip Unit Replacement

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bozosboss

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I am working a project to replace a Digitrip RMS 610 trip unit on a Westinghouse DSL power circuit breaker. The owner wants the trip unit replaced with an external, 3 phase, solid state relay with an inverse curve function. The RMS 610 currently utilizes both, Long Time and delay, Short Time and delay (LS) functions.
Has anyone had any experience with such a project? Any "pitfalls"? Is there an NEC or ANSI Standard problem with implementing such a modification?
 
Re: Power Circuit Breaker Trip Unit Replacement

Some things to consider:

1. Control Power - the trip unit is powered by the breaker CT's; external power is not required to trip the breaker. (It may be required to read a display.) The new external relay will require a source of control power, usually a DC battery system. AC control power could be affected during a fault. A capacitor trip device might solve the problem. Power is needed to operate the relay and to trip the breaker.

2. Tripping Means- the trip unit uses a flux transfer trip release device that requires very little power. It can only be wired to the breaker mounted protection module. You will need to add a trip solenoid to the breaker if it is not already equippped with an electrically actuated remote trip. This may require adding extra secondary disconnects to the breaker.

3. Current Transformers - the DSL breaker?s CT's for the trip unit cannot be wired to a normal relay. They are not 5 amp rated CT?s. You will need to add CT's to the breaker stabs and run them to the relay. Verify that there is enough room in the breaker cubicle to do this. Make sure to get polarities and grounding correct.

4. Ground Fault Protection - if the system is 3-phase, 4-wire and ground fault protection is needed , you will need to add a another neutral CT and make sure it gets wired in with the correct polarity on the correct circuit. (Sounds easy, but I have seen many done incorrectly.)

If the owner needs to do the conversion anyway to get the correct coordination with other breakers, look at just adding the new relay and not removing the trip unit. Set the trip unit's curves above the new relay curve as a backup. That way, you will still have protection if the control power fails or the trip circuit blows a fuse, and you haven't applied the breaker in a manner that might violate the breakers listing. Any special protection features of the manufacturer?s design (like a high set instantaneous) would still be in service.

BTW, when we have looked at changing the trip units to get better coordination we noticed that the extent of the power outage was the same with or without proper coordination. (Same load was dumped whether the upstream or downstream breaker tripped.) The advantage of better fault location was not considered worth the cost and risk of device change out.

Good Luck,
Bob W.
 
Re: Power Circuit Breaker Trip Unit Replacement

Excellent stuff Bob. Also, the RMS610 is also designed to protect the breaker. All of the trip parameters are set within the capabilities of the breaker, long time, short time, instantaneous, etc. It is possible that using an external means to trip the breaker may not assure that the breaker itself would be protected.
 
Re: Power Circuit Breaker Trip Unit Replacement

What is the reason to change out the internal trip unit to an external one? If the existing has LS adjustments, then you can easily adjust the characteristics to come very close to the characteristics of some external relay you may jerry rig. The trip unit is UL listed with the breaker, why install some untested combination?
I don't understand the benefit. The Digitrip is still a current product.

[ July 12, 2005, 09:32 PM: Message edited by: ron ]
 
Re: Power Circuit Breaker Trip Unit Replacement

And just think, the DSL with almost all certainty has a UL1066 listing and meets various ANSI and NEMA standards as an assembly with that RMS610 trip unit. I would like to understand that the individual who would like modify the breaker's means of tripping has taken this into consideraton.
 
Re: Power Circuit Breaker Trip Unit Replacement

Not really sure why the customer wants this done but URC makes a direct replacement trip unit for this breaker that will maintain the UL listing and is capable of being mounted externally to the switchgear enclosure. The unit is called an AC pro and is available with all the functions you need.
 
Re: Power Circuit Breaker Trip Unit Replacement

Scott,
I agree that the URC trip unit is probably UL Listed, but I don't think it is UL tested with the breaker as the original trip unit is UL tested with the breaker as an assembly.
 
Re: Power Circuit Breaker Trip Unit Replacement

how is the trip amps reset to a new value. Is thier a means on the breaker to do this. I also have one coming up on a square d i think that they will want the trip amps set on an 800 max breaker to I think 650 or so.
 
Re: Power Circuit Breaker Trip Unit Replacement

Ron,

Thats why we use AC Pro, unlike other units, the kits are specifically designed for each breaker type and maintain thier UL listing if you use the proper kit.
 
Re: Power Circuit Breaker Trip Unit Replacement

Stew,

I need more details to answer your question. The trip unit settings are what changes the trip values, you can also change the CT taps or rating plug depending on what type of breaker and trip device you are using.

If you do not know how to set up the unit, I recommend you have someone else retrofit your breaker or get soem additional training on CB maintenance.
 
Re: Power Circuit Breaker Trip Unit Replacement

With regard to this discussion nothing has been said as to what "additional" features the customer is looking for in addition to those that are available of the RMS610. If I'm not mistaken there is also a 710 and 810 trip unit available with more bells and wistles.
What addional advantages does the ULC trip have other that maybe perception and another player in the market?
 
Re: Power Circuit Breaker Trip Unit Replacement

Well the original post was looking for external mounting to the switchgear, and AC pro has that capabillity. It also has a 5 year battery and a better slit core CT design that dosent burn up like the other OEM's.

Hey I am not knocking the Digitrips, they are pretty good units, I dont know why you are getting so defensive, do you work for Square D or something?
 
Re: Power Circuit Breaker Trip Unit Replacement

zog, you made some good points with regard to why one would go to an external trip unit which were not evident before. It's not too unusual to have people replacing something because it's available and they can or the owner wants to because he same it somewhere or hade a sales presentation on it. I'm just not interested in seing money waisted on needless modifications because they are available and someone sold someone a bill of goods.
 
Re: Power Circuit Breaker Trip Unit Replacement

I dont really inderstand why they want to do this mod either, my guess is it is an arc flash issue. An externally mounted trip unit allows the operator to read the unit without opening the door therefore reducing the arc flash hazard. Good idea if you are already replacing the trip unit but I agree it probally isnt worth spending the money to do the mod if that is the only reason.
 
Re: Power Circuit Breaker Trip Unit Replacement

I not sure that is does, maybe there is a model that has that capabillity, but I cant recall ever seeing one. I know the 810RMS does but not the 510 or 610. I may be wrong.

The GE MVT+ does, but I am not sure about the Digitrip. There is an option for externally mounted metering but I dont think tripping indications is an option. Correct me if I am wrong
 
Re: Power Circuit Breaker Trip Unit Replacement

The DSL breakers are mid-80's vintage and originally came equipped with Amptector trip units. In 2002 the trip units were replaced with Digitrip RMS 610 units. According to the customer, the Digitrips have been unreliable (?) and especially so at low load levels (less than 10% rated sensor load) leading to nuisance trips.
The customer feels they can get greater reliability and availability by utilizing Schweitzer 551 relays on each breaker. The relays would have to be installed external to the breakers, in a remote rack, because of their physical size. In addition, a shunt trip relay will have to be added along with a breaker "a" contact, reliable trip power (probably from an existing 125VDC panel)will have to be used, wiring harnesses from each relay to each breaker (there are 8 breakers)will have to be added, new relay class CT's installed, and the remote relay mounting rack installed.
My biggest concern is whether this whole new breaker/trip unit configuration will now void any UL listing or testing of the DSL breakers. Is this going to violate any ANSI standard? I am wary of the whole concept and was hoping someone out there may have run into this type of request before.
Any thoughts or advise is appreciated.
 
Re: Power Circuit Breaker Trip Unit Replacement

Go with your gut, be wary of this one. Why are they running at such low load levels? If this is the norm they can just replace the rating plug with a lower one. Another possible problem is that at less than 20% loading (or somewhere arond there) the CT's are unable to power the trip unit and the unit runs on battery power, the batteries dont last long in that senario. I am not sure if a power loss to the trip unit would cause a trip but it might be worth looking into. Have the breakers tested by a company that specializes in breaker testing with some certifications.
 
Re: Power Circuit Breaker Trip Unit Replacement

Normally the breakers see a load of 60 - 80% of the sensor rating. However during periods where this part of the plant is down for maintenance or is in the "start-up" mode the load can be down to the 15 - 20% rang (or less) for 24 - 36 hours while the plant is starting up.
Yeah my "gut feel" tells me to be wary of this project. I may have to "eat" the time already spent on researching these issues and turn the project down.

To all who responded - Thanks for all the info. It pretty much reinforces some of my thoughts on this project.
 
Re: Power Circuit Breaker Trip Unit Replacement

If all xzy breaks loose, and it don't work, who goes to the court house? The guy who said I want it done, or the guy who did it!

What are they trying to say when they say Designated jail person?

I might call in sick that day.

Lw.
 
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