MCB Connections

NO.....What's an MCB?

Miniature Circuit Breaker.

Then definitely not...

Oh, why not? Let the child experiment.

I thought MCB was Molded Case Breaker? I'm confused. (SNAFU)

In industrial settings, MCB means Main Circuit Breaker most of the time, although the M can mean magnetic... or miniature...

Miniature only applies at amperages less than 100A.

Why would one want to do that?

i have a MCB incomer and 6 MCB outgoing feeders. and i wanna connect them using a comb busbar. so if its acceptable to connect it like i asked, then i can connect all the MCB in one row(Row DB) without any wires.

thats why i asked whether it is acceptable.

more like this circuit

http://1.bp.blogspot.com/_Dh5hCriDIus/S2-ZelQp8EI/AAAAAAAAAWg/OPtZk1nPegc/s1600-h/Electrical+DB+Picture.JPG

Simply follow the installation instructions that came with the equipment provided by the distribution board manufacturer.

We are the DB manufacturers. i just newly joined into design. my senior told me it was ok to do such connections. i was just curious about that and hence i asked you guys whether it is acceptable or not.

Does your senior know that you don't trust his judgement?

I can hear the conversation now:

Senior: "Yes Grasshopper, it's ok to hook these gizmos up this way".

You: "Wait, wait Senior, I'll have to check with my anonymous tutors on CR4, whom I've never met and whose qualifications I am ignorant of. They'll tell me if you are correct".

Go make yourself a new door Lyn.

regards,

grasshopper

Then why doesn't the organisation know already? Surely it has procedures and standards for this sort of thing? Surely it has done testing to establish those standards? Surely it has done customer feedback surveying to enhance those standards? What is the world coming to? <sigh>

Wow...that's not crowded at all...

I just don't like those...

Anyway...follow the manufacturer's recommendations...or suffer from your insurance company's lack of compassion when they won't pay out.

It is because in switchboards when someone do repairs/maintenance/new connections without switching off power, when the incomer is off it is safer to work on the upper portion for connecting new cables,disconnecting old wires etc even if power is available in the downside.

Yes you can feed in through the bottom of the MCB and distribute with a comb from the top.

An MCB is none directional in operation.

Using a comb is the neatest and most reliable way to bus out the power to each branch.

Proceed with confidence.

Trust your boss

BUT place a big warning label on the MCCB to warn the maintenance people that this breaker is wired in reverse of what everybody expect.

A Braille label preferably.

I hope it's acceptable, otherwise I've got 30+ years of switchboards I've installed to rip out!

AHAA! So yer the ONE!!!!

Keep it quiet. No-one will notice....

Most MCBs are marked 'Line' and 'Load' at the terminals. They MUST be connected as marked, otherwise they are likely to explode and maybe kill people.

If the MCB terminals are NOT marked, then you are free to connect line and load as you wish.

These being single-break devices, it is rather more difficult to clear a short circuit when the line is connected to the bottom terminal/s.

Why this elementary question ? One young engineer who asked this question in a MNC in India was shown the door. Rather cruel, but then the company was paying top salaries and quite rightly expected an engineer to know this.

Come on guys, the young ones have to learn somehow and from someone.

I can't think of a better place than here to obtain unbiased, competent answers along with excellent feedback/critique to affirm, confirm, or clear up confusion when faced with uncertainy.

What happened to "No such thing as a dumb question"?

What? You mean there is such a thing? Oh no, I have erred again.

Thank you all for the excellent forum, the good humor, vast knowledge base, wealth of information, and for taking the time to share your skills, intellect, and knowledge with this hillbilly.

HAVE A GREAT HOLIDAY SEASON!!

i agree that young engineers need to learn from experts. After retirement, i have spent a lot of time in training young engineers in switchgear design and peripheral fields. But this question is rather elementary in my opinion, since MCBs are clearly marked 'Line' and/or 'Load'. The question of connecting such an MCB the other way shouldn't have occurred at all. On the other hand, the IEC says that if the manufacturer does not want to restrict the connection, he must test it by feeding the power from the top as well as the bottom. So, no marking means one is free to connect it either way.

So, IMHO, the question is redundant.

It is my experience that the vast majority of AC miniature circuit breakers that are available down here in the antipodes can be fed from either end without any problem. This includes Schneider, GE, MG, Hager, GE, Allen-Bradley and some other minor brands.

I have seen DC-only MCBs that are fussy about which end they are fed from, but as other posters have noted these are always marked as such.

Is it true that they are installed the other way up in the southern hemisphere, so as to counteract the Coriolis Effect?

Depends on point of manufacture. If you'r in the northern hemisphere, and you import from the southern hemisphere, the MCB is installed right side down. and vice versa if from the southern hemisphere and importing from the northern hemisphere.

Or is it the otherway around?

Whether in southern or northern hemisphere gravity is downwards. Therefore top/bottom need not be changed.

Thanks for that advice. It's good to know that the information here is reliable...

If you are in Australia I have found that the MCB's do not specify the line & load sides on the MCB's (miniture circuit breakers) only on RCD's (residule current devices / saftey switches / earth leakage circuit breakers) .

I have seen the load & line sides in both directions on the same brand MCB's in many occasions i have not reciently seen any MCB's stateing any identification on the line & load terminals.

it is senceing current only so what differance will it make which direction of current flow is in as it is AC

It is somewhat more complex than that.

The MCB is a single-break device. The fixed contact is encompassed by the arc chute, and the arc products are deionised, cooled and ejected uneventfully when the incoming power is on that side...ie, when the fixed contact is 'live' or 'hot'.

If the power feed is from the bottom, the entire moving contact, the flexible connector, the trip system...everything is live/hot after the arc is quenched. Chances of restrike/flashover are much higher than if the top was live.

In some switchboards the outgoing MCB/MCCBs are mounted horizontally in which case what should be the polarity(in/out,left/right)?.

If Line/Load are not marked, it doesn't matter. However, it is a good practice to keep the fixed contact side connected to the busbar.

But, then, how this problem is overcome in modern "NO-LINE-LOAD-BIAS" MCBs?

By several small design improvements as needed....to wit :

1. Ensure no inter-phase bridging of ionised gases immediately after arc is quenched ... an example here (3 pole MCCB though)

2. When two or three MCBs are 'ganged' to make a multiphase one, the possible leakage path at the trip point (and any other vulnerable place) must also be blocked as well as possible.

There may be other small things i haven't come across.

Yes. That is how many residential and small industrial circuit breaker boxes operate. The feed comes in through the screw terminal of the large circuit breaker and that feeds the bus bars which the individual distribution breakers are attached to.

That was common practice on residential installations of Federal Pacific C/B panels about 50 years ago and is still done on some installations now, although modern panels have a dedicated master breaker.

Good Luck, Old Salt

No.

Yes you can do it. Technicaally there is no problem. But the good paractice is to follow the Manufacturer instruction and Incoming and out going Cables on recommended points and save the accidents may occur by third person who is maintaining the facility.

So whats the final verdict. "acceptable" or "not acceptable". some gurus say it is acceptable and some say its not.

Anyway i am going with "acceptable".

Fine Go ahead.

I know what I know and I know it is ok because I've always done it that way.

Or is it?

From a functional logical view point it is ok. Even with this directional arcing idea that was posted, which I don't buy. (except for DC apparently )

I tried to find a manufacturer's recommendation and couldn't find one except for DC applications where I found words that said line in and line out mattered.

The buss comb manufacturers said it could go in either the line in or line out sides of the circuit breakers.

Clipsal had a drawing which showed a main breaker fed from the top and a busscomb across the bottom to the branch circuit breakers with load wiring on top. Direction doesn't' seem to matter for branch circuits then? It still ended up neat with all wiring on top instead of the bottom.

I couldn't find an application note to say it was OK or over wise.

Did anyone else have a sniff around the internet and find anything concrete?

Pardon the typo....

.....carried out with connections required reversed when the terminals......

GA Sridhar with supportive data

Thank you.

Though the line/load polarisation is gradually disappearing, it exists in a small percentage of devices. Unfortunately, this aspect is not generally known, and manufacturers naturally underplay it. It may be good to publicise it a little i thought.....

Another thing to remember when working on circuits is to test them to make sure they are dead and without a charge before you work on them, even after you have tripped the breaker! Only takes some idiot to connect up wrong a junction box and you could have a circuit getting fed by two breakers!