MCB

Curiosity, what is ambient compensation?

Thermal bimetal relays use the bending of bimetals to indirectly sense overloads and trip. When the ambient temperature changes from say 10°C in winter to 45°C in summer in certain parts of India, the trip characteristic of the relay changes quite a bit. To prevent this happening, a passive bimetal is used to cancel out the bending of the active bimetals due to ambient temperature. Rather fascinating for a designer. Here is an image from a patent. The 10a are active bimetals, while 24 is the compensating bimetal.

i hope you are seriously asking ? That smiley at the end is a bit confusing ?

A GA for you means that I am quite serious. So, all of MCBs or breakers in general shall have this passive bimetal or it is an option?

Frankly, i have seen ambient compensation only in thermal overload relays used in starters. i have never seen one in any MCB or MCCB ! This Moeller one is a new one on me. i have not seen it.

If you think about it, why would one need ambient compensation ? If the product and what it is protecting are at the same ambient, would it not be better for the device to be un-compensated ? Since the protected cables and equipment would also suffer from the same high ambient.....

The only logic i see is that the protecting part of an MCB or MCCB would have a much lower thermal mass than the cable and the load, so a bit of compensation would allow the load to run without nuisance tripping. In IEC 60947-4, a little bit of under-compensation is specified. This is achieved generally by making the compensating bimetal a little shorter than the active bimetal ...

Hey, somebody stop me please, before i bore everybody

KVS! That exactly is what has been baffling me! My thermal overload realy is in a different ambient tahn the motor which it is protecting. Then, in such a situation, what is the point in providing ambient compenstaion? For example, I can install my MCC in an Airconditioned room, wherein ambient temperature compensation would delay the tripping for a given overload, whereas my motor can be near a boiler or furnace an thus might need quicker tripping.

Exactly MCCs are quite often in controlled ambient, motors never are !

So, i think the ambient compensation is (a) an ego-trip for the designer (how nice it is to make the relay adhere by and large to the median of that nice under-compensation per Fig 7 of IEC ... a triumph for the designer !) and (b) a good sales point !

Nevertheless, wiser minds than mine have decided that ambient compensation (under-compensation ≈ 0.4% per °C if i remember the words of a master designer at Bedford, England) is a good thing. So, there we are.

MCBs and MCCBs are much cruder devices, meant for coarse protection.

The thermal overload motor protection relay is a masterpiece of design, providing overload and phase-unbalance protection, with ambient (under-) compensation, sensitive tripping, all in a package not more than $2 in cost ... (now maybe $3 ?)

electricalexpert65:

Does it has any information of on the OL relay? How I can recognize that? It could come with operation temperature, let's say 30 to 40degC?

No! Ambient temperature compensation feature is not marked on the relay. But, it has almost become a standard feature with all reputed switchgear manufacturers. One must only take the word of the manufacturer.

Or, you can subject the relay to testing under different amibientconditions and verify if the trip times are consistent.

I have a doubt regarding your answer.MCB manufacturing concentrated in one place but consumers are different places(working temperatures are different) .So the calibration test of MCB's at manufacturing time can be approve ???

It would be helpful if we know a little more about you...are you in India, are you working for a company which makes MCBs and RCDs, have you studied IEC 60898, so on. Any product cannot be learnt in a series of sporadic posts on CR4. But we can get almost there if you do your own learning, and then ask specific questions. By the way, have you downloaded the Moeller paper and studied it yet?

Yes am from india, working in a prvt company.We planned to make switch gears but lack of knowledge .Am trying to study about the same.In my hand IS files of MCB & RCCB are available, but some times it didnt meat my requirement.So that time I need help from CR4 engineers.

Standards won't tell you how to design the product, but knowledge of standards is a must for the designer. You should learn about the MCB and RCCB by searching the web, (including other threads of CR4), dismantling and studying samples of other companies etc. You will need a long time if you want to learn piece by piece on CR4.

Your company can also retain an expert as a consultant to help you desing such products

MCB- IS/IEC 60898-2002 & RCCB- IS 12640, this standards are in my hand.I am using web ,but some times didnt get clear explanation.So that time i met my requirement by the help cr4 engineers like you & ee65 etc.Other brands studies are going .

We will surely help whenever we can. However, it is a massive job if you don't know the subject.

For example, there are (a) the contact zone, which does the job of making, carrying and breaking any current including short-circuit current, (b) the arcing zone, with the arc chute and the de-ion grids (there will be 13 grids or more, do you know why?) (c) the tripping system, which includes the bimetal and its heater, and the magnetic system, and (d) the mechanism, which is trip-free and sometimes quick-make (always quick-break when tripping)...

Then you should acquire knowledge of the materials used, plastics, metals, silver-alloys for the contacts.

Long job, provided you can find someone to teach you. Best across the table.

i wasn't joking when i said your company can hire a consultant. When you have no internal knowledge, this is the way to go.

Some of other models I am studied but i cant see 13 chutes only 12No,do you know why ?

How does area of arc chutes opening depends quenching of arcs ?

Of course i know why. And i get paid for explaining why, i am a consultant you know

Joking apart, you need to study this paper to know about the arc, and how it is quenched : ect154

In short, there is a cathode drop of approx 20V per grid. So, 12 grids will give rise to about 240V of recovery voltage which will help in quenching the arc. One more is prudent. So i said 13 grids.

You see what i mean? A person who doesn't know this may eliminate a couple of grids to save cost, make the product more compact, whatever..... disastrous !

KVS! Is this 13 Grid Plates tru for ACBs too? I have seen about 29 plates in M-Pact ACB, if I remember correctly.

Oh yes, that old reliable had too many. If i recall right, Schneider (and maybe ABB Emax and perhaps U-power also) have 16-18 grids. i guess here, the aspect of cooling of the arc takes higher priority, so more mass of steel is there. You can perhaps ask someone on that team. Here is a Xn view of Masterpact.

hello kvs have a nice day,

i have some doubt regarding MCB Characteristic curve, why should we choosing magnetic tripping rage 5 _ 10 times rated current and thermal tripping range 1.14 _1.45 times of rated current.This rating depends which factor.Can you help me................

MCB trip characteristics are classified as B,C and D. These refer to the magnetic trip, which is 5-10 times the rated current.

This part of that article is relevant...

Take a look at ABB Emax catalog for related data.

http://www.clrwtr.com/PDF/ABB-Controls/ABB-Emax-Power-Breakers.pdf

Thank you. But this is a commercial catalogue of Emax. Doesn't have a cross-sectional view or anything like that. Thanks for the thought anyhow.

You will note that EE65 and i were discussing the number of de-ion grids in the arc chute of an ACB. Schneider Masterpact seems to have 16 or so. So has ABB Emax. Here is an image from one of the patents on Emax.