MCCB vs HRC overcurrent protection

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Discuss MCCB vs HRC overcurrent protection in the Electricians' Talk | All Countries area at ElectriciansForums.net

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baldsparkies

Have been looking at a situation where a 400 amp MCCB is feeding a ups. There is a mains supply to a genny changeover switch, along with the genny supply, and the MCCB is between the genny switch and the UPS.
Yes its a big bu##er.

Any ways there was a momentary power fluctuation which threw the mccb, great because the change over would not have seen this, cus the main supply would still be on to it.
That meant the genny didnt kick in and walla a problem once the ups powered down.

Now my question, MCCB's are notorius as we know for kicking out when hit by surge current as probably happened here. All we needed to do was to re set the thing. but of course the damage is done by then.

What do you clever types think about replacing the MCCB with a 400 amp switchfuse with HRC links instead of the MCCB. Before you ask, the thing is dialled up to max, Load test shows 280 amps running full load.

I reckon its the best way forward but wanted to pick yer brains
The install is top notch. All composite panels, monitor equipment PFC installed, should be a dream if not for this one week link in the chain.

UPS supplys are all individual 150mm tri rated cables, so nice and bendy to work with.

Thanks for your input in advance. ;)
 
AS said MR E, if not for this one weak link.
Change over switch just aint going to see a loss of power on the load side, only the supply.
And even if the genny did kick in, the mccb is still out so the supply would stop at that point,

Anyway I digress, Can HRC's cope with surge better than those blooming MCCB's ????? I reckon yes but your thoughts appreciated.

By the way, I have never seen a supply change from mains to genny in milli seconds, Thats where the ups comes in handy, gives enough time for the genny to fire up.
 
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I would think about your proposal to replace an MCCB with a Switch-fuse.
Switch-fuses have there own inherent disadvantages, one important aspect is ''phasing'' where one of the 3 fuses blows... They are also expensive to incorporate any control mechanisms!!

Why do you say a 400A MCCB is wound up to it's highest setting, when the full load has been measured at just 280A?? Are you sure this is a 400A breaker or is the 400A just the frame size?? Can you tell us the manufacturer of these MCCBs?? One other question, ...Are these MCCBs, Changeover Switches, etc, mounted in a common Switchboard panel??
 
My guess is the setting has been adjusted up to max because the same problem occured 18 months ago. They get a blip in the mains, not an outage, and its tripping the mccb.
I have been to office blocks and factories over the years were there has been power failures. When the power reinstates, some but not all of the mccb,s trip out. I put it down to the loads they supply are coming on line in one hit. I do take your point on phasing, but the equipment supplied is all single phase. Really I am just wondering if HRC's are less prone to blowing than mccb,s to tripping. As an example the DNO cutouts always have HRC's. When power fails its rare for these to blow when the supply comes back in. Oh yes almost forgot its a 400A mccb for sure calibrated from 160 to 400 amps.
I need to get more detail cus this all occured at 2.30 in the morning, so not at my best. There must be an answer to the problem somewhere.
 
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HRC fuses are used by the DNO because fuses present the cheapest method of providing PSCC protection to an installation...

From what you describe, as in the ''Blips'', This can easily be protected against by provision of Surge Protection.

I don't know of any manufacturer of MCCBs that have a setting range on a 400A breaker of 160A to 400A!! I think that your 400A is a frame size, and that the actual size of this breaker is in the region of around 280A to 300A, depending on manufacturer.

Are you saying that this breaker (MCCB) is single phase, or just that the final loads are single phase?? If it is the latter, the UPS isolation transformer along with many other components in the UPS will be 3 phase and will not like at all, a situation where it's supply is single phasing!!!

If you can give a little more information, after you have been back and assessed the situation on this installation it would be helpful, and a pic or two even more so....
 
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Many thanks 54, Will check the frame size,
You are probably correct in that the 160 to 400 is reffering to this rather than the calibration of the device.
The information received is second hand, over the phone regarding details.
We re set the mccb with no problems. They can offer a fair bit of resistance before latching in as you will know.
The loadings were around 280 per phase fairly well balanced but it IS a 3 phase supply feeding single phase loads on the load side of the ups.
We need to isolate and remove the cover, because apart from what was readily visible, and conducting a load test whilst there, The rest of the info was coming from the O&M manuals, which they found the following morning.
We are going back up during the day, so will get a better assesment of the situ then.
Many Thanks for your input.
PS. Point well taken regarding single phasing, not a road to go down.
 
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Can you adjust the time lag coefficient on the MCCB? A small mains blip shouldn’t cause nuisance tripping.
What’s it going out on over current or under voltage? I had a problem with U/V on an ACB a few years ago, 2ms delay solved the problem.
 
Tony said:
Can you adjust the time lag coefficient on the MCCB? A small mains blip shouldn’t cause nuisance tripping.
What’s it going out on over current or under voltage? I had a problem with U/V on an ACB a few years ago, 2ms delay solved the problem.
Click to expand...

Many thanks Tony,

I will have a look at the calibration settings when on site and take it from there.
Some units have 2 dials covering trip threshold marked In and Io, others are even more flexible with 3 dials, the third covering Isd which I believe is the prospective fault current.
i usually multiply the nominal rating set on dial 1 Ir by the Io and Ir calibrations.
Any other info would be much appreciated, but my guess is they have been gradually increasing loadings, and as a result a loss of power, even though momentary would mean a sudden surge through the device which would re instate under full load is causing the tripping. So hopefully re calibration could well be the answer. They also need regular checks on the system to ensure max demand criteria remains within the design parameters of the install, but you would think that would be common sense.
 
That old chestnut again, has anything changed? No! Well, we added this and that and oh, that other bit, but nothings changed! (Well it’s only bits of stuff).

A different scenario but I had to up rate an ACB from 800A to 1200A and add a 3rd feeder cable etc. But nothing had changed! Someone sneaked the 300HP compressor in under the cover of darkness!
 
Aint that the truth Tony, Often with the bigger companies, they have various contractors adding here and adding there with no one keeping check on who's doing what, where and why. I remember on one site Red phase had 80 amps Yellow 260 and Blue 140. But it happens all the time, at least on this one the phases are pretty well balanced so no issues there. Thanks for the responses to all.
 
gaz101 said:
Is it possible to add an alarm
To call maintenance if the mccb tripped
Click to expand...

It’s a bit late by then, the problem has to be stopped before it starts.

Quote the plant manager “did you know the main breaker’s tripped again?” Me “yes the fu**ing kettles stopped boiling and it’s bl**dy dark in here!”

Discrimination and trip settings are a nightmare when you get to 400A+ breakers, as Baldsparkies obviously knows. Both time and current are adjustable so it’s easy to alter one setting on the local breaker and move the problem one step up the ladder.
 
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baldsparkies said:
Aint that the truth Tony, Often with the bigger companies, they have various contractors adding here and adding there with no one keeping check on who's doing what, where and why. I remember on one site Red phase had 80 amps Yellow 260 and Blue 140. But it happens all the time, at least on this one the phases are pretty well balanced so no issues there. Thanks for the responses to all.
Click to expand...

Very true mate. the sites im working on, nobody has a clue whats in them and what isnt. half of them are not even fully functional, and trying to find out anything ie teste results ect, is a nightmare.

at the moment, they have employed another frim to carry out PIR's, but they are also testing our firms half finished installs, failing them, as they are not complete, and ive still got to test them when they are fininshed. i reckon that the firm will bill for remedials, and when thhey turn up to carry out the work, wuill find it all kosher, cos we would of finished, and earn for nout.

its a shame that electrical safety isnt a priority.
 
ill try and get some photos of some of the gear i see. its ancient, but sill works. in one place there are 2 massive generators, one english electric, and one blackstone. they have DC generators at the end, on the same shaft, which i guess are being used as exciters. a small petrol engine acts as the starter. its a shame, but both of these beasts are no longer operational. the panel for the blackstone genny must be 50's, and still in good nick. ive opened some of the cubicles, just tp noes at the wiring, and its the neatest ive ever seen.
 
Would be interesting to see Johnboy. The fact there still operational is testament to the quality of build with the old stuff,
I mean look at me. Born in 1953 and still going strong, keep tripping over the zimmer frame though.:D:eek:
 
I was heart broken to take this out and replace it with modern crap. Just a pitty I didn’t get a picture of the front, polished black slate

View attachment 5718

We even had a mercury arc rectifier.

Meet Rodney

View attachment 5719

Used to scare the living daylights out of me at first, but then I got quite fond of it. To watch it working was like watching a living thing.
 
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Engineer54 said:
HRC fuses are used by the DNO because fuses present the cheapest method of providing PSCC protection to an installation...

From what you describe, as in the ''Blips'', This can easily be protected against by provision of Surge Protection.

I don't know of any manufacturer of MCCBs that have a setting range on a 400A breaker of 160A to 400A!! I think that your 400A is a frame size, and that the actual size of this breaker is in the region of around 280A to 300A, depending on manufacturer.

Are you saying that this breaker (MCCB) is single phase, or just that the final loads are single phase?? If it is the latter, the UPS isolation transformer along with many other components in the UPS will be 3 phase and will not like at all, a situation where it's supply is single phasing!!!

If you can give a little more information, after you have been back and assessed the situation on this installation it would be helpful, and a pic or two even more so....
Click to expand...

Just an update, It is indeed a 160 to 400 amp MCCB Merlin Gerin NS 400H.
Phoned Schneider Technical today, and they were very helpfull, with all the info I needed to have a little play with the blue dial thingy's. :)
I also made sure the mother wouldn't tripp under load, consequences would not have been good :eek:
6 calibration dials in total marked Io Ir tr Isd Ii and tsd. So plenty to play with.
Anyway, all settings were set to minimum which is the manufacturers default settings, not maxed out as I was told (Just goes to show).
Only one to watch for is the let through current on the pfc, but other than that all good after recalibrating. (He's lying, he chucked it away and used the 6" nails :D)
So all done and another happy customer, Thanks for all the replies and advice lads, much appreciated.
 
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hi baldsparkies. is there any chance you could elaborate on the advice you received, so that i could learn a little too please?
what do all of the letters mean? ect ect.

thanks mate,

john
 
johnboy6083 said:
hi baldsparkies. is there any chance you could elaborate on the advice you received, so that i could learn a little too please?
what do all of the letters mean? ect ect.

thanks mate,

john
Click to expand...

Here goes Johnboy, I will try to sound intelligent :)

Io and Ir set the overcurrent tripp threshold, with In being the nominal rating of the MCCB
So if In is set at 300A Io at 0.7 and Ir at 0.8. you multiply 300x0.7x0.8 to give you 168 amps.
Isd sets the let through current for the pfc under short circuit conditions.
And tr and tsd set the lag times respectively.
All in all very much like the 3 dial set up units with the luxury of adjustable time lag, (Still doesn't beat the 6" nails though):);)
 
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baldsparkies said:
Still doesn't beat the 6" nails though:);)
Click to expand...

6” nails? Go for the no blow specials!

I had a 4000A breaker fail, the isolating mechanism jammed when I tried to get it back in to service. So in desperation I used the old short it out method. 2am is not the time to be looking for old copper bus-bars but I managed to find 10’ of 2½ X ¼” bar and bridged the ACB out. I had to limit the power to just 3000A as I couldn’t find enough copper to allow full power through.
 
Nice to see a hands on thinking sparkie out there Tony,
Ever thought about Astronomy ?? They still haven't decided on the big bang theory, lol.

By the way, one thing I did have verified was that under load adjustment with these units is possible. You are safe dialling up ie increasing but NOT down !!
If your load is reliant on a constant supply (Mine was) then downward adjustment could cause the device to tripp, if you dial below the tripp threshold.
Might be usefull to some of you guys one day if found in the bricking it situation.
 

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