Testing Thermal Overloads 2

[jlatta]

At the plant where I work, we periodically test some important thermal overload heaters in place (in the MCC). I want to make sure we're doing it right, so does anybody know if there are "standard" test methods with acceptance criteria that take into account the temperature difference between the MCC and the assumed 40°C ambient the overload heater curves are based on? These 480 vac MCCs are generally in a controlled environment with approximately 25°C ambient.

 

[jraef]

What type of overload relays do you have? If they are bimetal, you may want to rethink the practice of periodic testing if it involves current injection to cause a trip. Bimatal strip build up a "thermal memory" in that they do not return to 100% of their original state. After repeated trips they will tend to trip earlier and earlier until they become a nuisance. Eutectic Melting Alloy overloads are less susceptable to that problem, but are not the best solution. If the system is critical and you want to have frequent test records, I would suggest going to a solid state overload.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jlatta]

Thanks, jraef. Yes, they are bi-metallic. Mostly ITE-Gould "G30T" style heaters. The test frequency is only about once every eight or ten years.

 

[jraef]

Oh I would not call that frequent at all! Holy cow, you have someone around that will keep records that long and perform a routine maintenance task that hasn't been done in 8 years? I'm jealous. Bravo!

Back to your original question then, there are current injectors used typically by those who test circuit breakers and protective relays for a living. They are quite expensive and may not be worth it for occasional use, but if it were me, I would hire one of them and watch what they do and what they use to do it. Then I could fairly evaluate whether it was worth trying to do myself. I have found that the high cost of the specialized equipment is difficult to justify against the relatively low cost of bringing in an outside tester.

Here are a few links from when I last investigated this. I was looking for full protective relay testers, but these same compnies may have less expensive versions for current only.

http://www.adwel.com/relay_test_set.html

http://www.avointl.com/us/products/ProductDetails.php?ID=202&Description=

http://www.mantatest.com/prod_1710.html

"Venditori de oleum-vipera non vigere excordis populi"

 

[busbar]

 
And another test-set pair is

http://www.phenixtech.com/products/Circuit_Breaker/brochures/30105-HC1-HC2-PORTABLES.pdf

 

[UKpete]

My employer manufactures their own electro-mechanical thermal overloads for their MCCs. Included are the bi-metallic strip types.

We simply test them with a variac feeding a current transformer whose output is connected across the overload. I don't have a great understanding of CTs but I presume it is used in the opposite direction to their usual use in monitoring currents, i.e. we feed into the secondary and get a large current out of the primary. The output current is measured on a calibrated clamp meter and the published time-current curve is checked on one or more points (depending on the particular type of overload). The curve of course has a upper and lower tolerance band used as pass/fail criteria.

The same overloads will detect single-phasing (I'm not sure how this works), we test for this but there is no published pass/fail criteria.

 

[busbar]

For 120V 60Hz, a place to start experimenting may be a 600:5 C200-class [beefy relay-class] window current transfomer and some welding cable. Minimize secondary reactance by keeping larger cross-section loops on the secondary circuit. {This becomes apparent after a little experimenting that DC resistance in the secondary loop is much less important than reactance.}

Assorted components can be found surplus in some areas.

Current transformer ~600:5 C200

http://www.geindustrial.com/products/specs/686.pdf

[wt. >47 lbs]

A 126- or 136-series Powerstat

http://www.superiorelectric.com/PDF/POWERSTAT-c.pdf

[wt. ~36 lbs]

 

[jlatta]

Thanks, Busbar. This discussion is starting to get away from my original question, but it's still interesting. We're a power generation facility and I think our techs use Doble relay test equipment to inject the test current directly into the overload heaters. The original question I asked was about how to take into account the temperature of the heaters under test when they weren't at the 40°C, since the manufacturers' curves generally assume a 40°C starting point. I also polled other members of my industry group, and the best answer I got so far is to preheat the overloads to ~40°C by injecting 100% FLA for 10 minutes as a "no-trip" test. I liked that because in our industry (nuclear power) the main reason we're testing them in the first place is to verify they won't trip early at the wrong time and stop a safety valve from re-positioning.

 

[busbar]

 
Very generally, bimetal-type motor-overload relays have long been marketed as “ambient compensated,” so temperature should not affect trip times. This is not supposed to be the case for eutectic-alloy relays.

The overload “class” [10-20-30] is supposed to be the time in seconds that the relay operates at 500% of rated. Anything more specific would have to be furnished by the relay manufacturer as a model-dependent time-current characteristic curve [or “band” as the case may be.]

 

[jraef]

jlatta,
One thing you probably should know, in the rules for mass producing overload relays, the Trip Class is the MAXIMUM tripping time, but it is considered acceptable it they trip earlier than that. In other words, if avoiding nuisance tripping is your goal you may not like what you find.

Minor correction Busbar: the OL Class is max. tripping time at 600% current, not 500%.

"Venditori de oleum-vipera non vigere excordis populi"

 

[busbar]

jraef, my mistake. 600% so noted.

 

[jlatta]

Thanks busbar & jraef,
Yes, I had found the Trip Class 600% maximum rating in previous Google searches, too. And it explains why here at the nuclear plant we test to verify the overloads won't trip early for the safety valve motors, and why we pick the heaters for them two sizes higher than normal in the first place. The overloads we use aren't advertized as “ambient compensated” as far as I know. So, at the risk of becoming the local thermal overload "expert" I would like to ask just one more question: Generally, how does ambient compensation work with the bi-metal thermal overloads?

 

[jraef]

In Bimetal relays, a bimetal strip bends with heat to push a trip bar that activates the spring-loaded mechanism. To compensate for ambient heat conditions near the relay, that trip bar is itself suspended on a bimetal mechanism that moves slightly at the same rate as the bimetal strip, but since it is unrelated to current flow it only bends based upon ambient heat that might also be affecting the current sensing element. So the distance required for the bimetal strip to move for activation is kept constant regardless of any ambient temperature already in the relay itself. The additional bending of the current sensing element that causes tripping is then a result of heat generated by motor current only. This by the way is why you cannot do this with a eutectic melting alloy OL relay.

"Venditori de oleum-vipera non vigere excordis populi"

 

[mc5w]

The Klockner Moeller catalog has a diagram of how single phasing protection in a bimetallic relay works. You have 2 sliding bars going crosswise to the bimetal strips. One bar has studs that are pushed by the hottest bimetal. The other bar has studs that are pushed by the coldest bimetal. the 2 bars are joined together by a level that pushes against the snap action for the contacts.

By the way Telemacanique also makes bimetallic overload relays without single phase sensitivity.

Mike Cole, mc5w@earthlink.net

 

[jraef]

mc5w,
Nothing wrong with what you said but for the benefit of others who might be confused, part of what you described was an answer to a question not asked. That is a description of how the differential trip mechanism works on IEC style OL relays to help prevent single phasing of 3 phase motors. He asked about ambient compensation, and had mentioned earlier that they are old obselete ITE-Gould (NEMA style) bimetal relays, not IEC of any sort. The differential tripping issue is not applicable to US made bimetal OL relays, they never had it until much later when they learned it from the EU mfgrs. By then, ITE-Gould was long gone.

"Venditori de oleum-vipera non vigere excordis populi"

 

[ScottyUK]

Hi jraef,

Before you jump too hard on mc5w's post, on 3rd October UKpete had commented in passing that he wasn't sure how single-phasing detection. Perhaps this was the reason for mc5w's explanation?



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[jraef]

Oops, I missed that, you are right ScottyUK. My appologies mc5w, although I was not really intending to "jump too hard" on you on purpose. In re-readng my response however, I can see how that could be interpreted as such.

The reason I posted that is that I find this forum to be used quite a bit by people who are just beginning to learn things that many of us take for granted. Although most of us probably understand what you meant, some of the new people might take that information and assume, for example, that ALL bimetal OL relays provide single phasing protection, which is not true, especially here in the US.

"Venditori de oleum-vipera non vigere excordis populi"

 

[UKpete]

mc5w/jraef, thanks for your comments. I'm a "semi-retired" engineer involved in testing, not involved in design any more but still interested in the products. I didn't strictly address the original post - we don't make any compensation for ambient temperature, I will have to find out if our overloads are self-compensating.

Regards

UKPete

 

[mc5w]

UKPete,

Have you found the diagram in the Klockner-Moeller catalog as to how single phase differential works.

Part of the purpose of the single phase sensitivity in an IEC style overload relay is that it is possible for a motor to run partially single phase. One of my customers had a motor burn up because a corroded fuse clip caused the motor to run partially single phase.

After rewinding the motor the motor would not come up to speed and tripped the motor overload relays after 2 or 3 minutes without any load on it. I was called in and found the problem.

Cleaning the fuse clips and blades with #220 or #240 silicon carbide paper and then putting in some Ilsco Deox(R) electrical grease fixed the problem.

 

[UKpete]

mc5w, I have looked on the US Moeller site and couldn't see anything relevant. The UK Klockner site does have plenty of catalogues but they are a bit large and numerous for me to download.

However, I have since dismantled one of our bimetallic overloads and had a good look. Basically it looks similar to what you describe. I'm still not sure whether we have ambient temperature compensation though. We usually test at 2 x rated current with a stop watch, the factory temperature is pretty constant all year around.