Circuit Breaker Fused Disconnect ?

[snoogie]

Does anyone have any thoughts / experience with using circuit breaker type disconnects over fused switches?
I have used both in the past but always worry about fault levels when using cricuit breakers as it is often very difficult to estimate actual fault currents.
My applications are typically AC/DC Drive panels and general PLC systems up to around 600Amp for North American market (NEC, CSA). I have yet to see a circuit breaker type diconnect explode (but have seen smaller MCB devices do exactly that.
Snoogie

 

[ScottyUK]

Snoogie,

I'm in Europe, so excuse my non-North American terminology.

What sort of magnitude do you mean by 'fault' current? Most MCCB type circuit breakers are capable of clearing between 35 and 65kA. I cannot imagine any PLC or LV drive panel having anything remotely near that level.

One of the things I struggle to convey to our guys is that when you close a switch-fuse onto a fault, keep pushing even if the lights go dim and it hums like crazy. Allow the fuse to break the fault, don't draw the arc onto the switch contacts. It takes a lot of presence of mind to do this, rather than trying to pull open the switch, but the consequences of drawing the arc onto the switch contacts are potentially lethal.

A circuit breaker is pretty much guaranteed to break a fault within its rating, although that certainly doesn't mean it will be fit for service afterwards.

Your choice should consider the level of training and experience of the people who will operate the equipment.

 

[snoogie]

In North America very large supply capability equipment can be hooked up to your panel. My concern is not with the PLC /Drives pulling these sort of values but if we have a ground fault on the buss bars or cabling inside the panel then potentially the full KA fault current of the supply system could flow throw the MCCB and desintegrating it because it just cannot take 100KA, Where as with fuses they can handle up to 300KA or more.
I used to work in the UK and typically we would use 50ka MCCB,S buy always had a some concern and would tell the customer that the max KA capability of the system was whatever we used for MCCB.

Snoogie

 

[ScottyUK]

Hi Snoogie,

100kA at 415V (460V?) is pretty high. A 2.5MVA transformer gives about 70kA at the terminals for a typical design. How big is your transformer and what is it's impedance? I guess your drives must be almost on top of the supply transformer to get that sort of fault level. The impedance of, say, 50m of feeder tends to calm things down a lot.

Have you considered using an ACB with a fast electronic tripping unit? With that sort of fault level, I think the ACB has a higher survivability than a moulded breaker. Fault-limiting fuses with a switch rated for fault-make / load-break are well worth considering. Remote switching is a better option - eliminate the risk to personnel by moving them away from the switchgear.

 

[snoogie]

I think I am being overcautious when you look at the size of the transformer required to produce that sort of fault level, but then agin this type of equipment does go into steel plants etc. So big supplies are possible.

Thanks for your thoughts i feel more confident after this chat!

Snoogie

 

[jraef]

Hello Snoogie,

You will always find disagreement in this issue. It is a discussion that predates us all and will likely continue after we pass on. The best you can hope for are reasonable opinios from which you can support or dispell your own experiences.

I tend to favor breakers. I have seen the films put out by Bussman showing exploding breakers, and in fact I have witnessed same. Nonetheless, I have seen many many many more applications where the wrong fuse or a piece of pipe was used because of a lack of availability of the proper replacement, and the consequences are equally devastating. The likleyhood of single phasing a motor is also a concern to me, as is the arcing issue mentioned by ScottyUK. Breakers lessen the chances of misapplication and are designed to prevent the other issues.

Quando Omni Flunkus Moritati

 

[ScottyUK]

Hello jraef,

I agree with much of what you say - breakers are the best choice for the majority of applications, especially if untrained or semi-trained people will operate them. Fuses win out for ultra-high fault levels beyond the range of (European) breakers, or on heavy DC loads. I've seen a couple of breakers blow up breaking a DC fault in an inductive circuit. What a mess.

Do you know if the Cooper-Bussman films are available in Europe?

 

[jbartos]

Suggestion: To feel safe around circuit breakers, it is necessary to calculate short circuit currents for the power distribution upstream and down to the circuit breaker bus. The calculated short circuit current must not exceed the circuit breaker short circuit current rating also known as interrupting rating or interrupting capacity. It is a good practice to have a reasonable design margin left, e.g. 15% or so.

 

[ScottyUK]

jB,

Have a look at the following link:

http://www.bussmann.com/services/training

The file you want is titled "Interrupting Rating" under the heading "Overcurrent Protection Seminar Presentation Topics". See slides 32 - 39 for a 35kA rated breaker interrupting a 25kA fault.

Do you still feel safe around circuit breakers?

 

[PUMPDESIGNER]

It is best to consider the safety issue as a whole and not focus on just one aspect unless that aspect requires top priority in a specific instance.

Example:
If an MCCB were directly fed from a huge low impedance transformer, close to the MCCB, and capable of delivering 100,000 amps of fault current, then the AIC rating of the MCCB immediately climbs to the top of the priority ladder.

Whereas, if that same situation existed except that the transformer was hundreds of feet away and the MCCB was not fed directly from the transformer but instead was fed through another protector, then the AIC rating would not be so important because the fault current is going to be much less and in reality the devices upstream are responsible for the total AIC.

Circuit breakers provide greater safety in our applications (pumps) due to their ability to fully disconnect the load under fault conditions, which fuses almost never accomplish. The vast majority of faults are low level, not high level, so very rarely will the full available fault current occur. So in the vast majority of situations, circuit breakers fully disconnect any fault whereas fuses do not provide that.

There are two policies we have to protect ourselves:

1
We never allow our equipment to be connected directly to a main supply transformer, the owner must always provide his own high level AIC protectors near or at his service entry if the potential for high level faults exists. This seems sensible as it puts the onerous requirement on the engineers designing the service to ensure that if a possible high level fault occurs the protective devices nearest the service are going to intervene.

2
We standardized on a higher level of AIC rating for our circuit breakers, and we do not sell any product without that higher AIC rating.

We realize that if the worst scenario possible develops we may still have a problem. But lets all admit one thing, it is exceedingly difficult for a bolted fault to occur, and then add that to a possibility of that type of fault occurring in a situation where the fault current can even reach the strength required to cause a problem.

A tremendous number of things have to line up just so in order for the worst thing to happen: Bolted fault occurs, and at just the wrong point in the sine wave, and the device is very close to large low impedance transformer, and the wire size from transformer is huge (not usually the case for economic reasons), and there are no other devices upstream to cause resistance, and the cause of the bolted fault must be huge, like a solid bar of 1” steel falling across a huge buss (the initial cause of the short usually blows up and disintegrates before the current can even begin to climb too high).

Does this ever occur – Yes.
However people are hurt every year messing with fuses and being shocked by partially disconnected loads, probably far more than are ever hurt by explosions from bolted faults.

I realize this is just my opinion, but I face this every day and when people default to fuses because it makes them feel safe I cringe because they are not taking all factors into account and they do not really accomplish higher safety but lower safety in many situations.


PUMPDESIGNER

 

[jbartos]

Comment on ScottyUK (Electrical) Dec 3, 2003 posting marked ///\\jB,
Have a look at the following link:

http://www.bussmann.com/services/training

///Thank you. A very impressive site and pictures. It appears that the circuit breakers have been grossly underrated for the testing and demonstration purposes. Most people trained and educated in electrical disciplines are very familiar with a danger due to underrating protective devices including fuses. Low interrupting rating of a fuse is also disastrous. Unfortunately, it was not shown on the Bussmann Fuse website slide show.\\The file you want is titled "Interrupting Rating" under the heading "Overcurrent Protection Seminar Presentation Topics". See slides 32 - 39 for a 35kA rated breaker interrupting a 25kA fault.

Do you still feel safe around circuit breakers?
///I feel safe around circuit breakers just like around fuses. No big difference. They both have to be properly designed or selected with sufficiently designed interrupting rating including a design margin about 15% or more. I have worked on many diversified projects; where there were no fuses specified at all, e.g. buildings, ships, trains, power plants, process plants, substations, machinery, infrastructures, etc.\\

 

[ScottyUK]

jB,

The breaker pictured in the link had a safety margin of 30% on its interrupting rating (35kA capacity breaking 25kA). The firework show isn't so unusual with a heavy fault, but it makes a strong case for enclosing breakers in a robust metal enclosure, and keeping the door to such an enclosure closed.

 

[snoogie]

Thanks guys I like that web site too their electrical protection handbook is good to (and free) or mine was anyway but i am not sure how old it is.

I think I maybe leaning towards breakers now, however another thing i noticed was that you have to watch the rating with respect to a fault on just one phase (which I guess is more likley) as the rating is very much reduced for individual poles.

Snoogie

 

[jbartos]

Suggestion: One weak aspect around low voltage molded case circuit breakers is that they do not have any refinement between symmetrical rms interrupting rating and asymmetrical interrupting capability (close & latch) similar to medium voltage switchgear. There might have been an asymmetrical fault much exceeding the molded case circuit breaker rating. In that case, the molded case circuit breaker is shattered by electromagnetic forces.
The side show does not elaborate on this aspect, unfortunately.

 

[skiier]

Snoogie. Jbartos and scotty uk have some great points. The simplest thing for you 2 do with any installation is to do a fault calculation and determine the available fault current level at the line side of your cabinet disconnect. Have the customer spec it or some outside agency that specializes in it. I think that you will find in 90% of cases that the feeders (conductors) to your cabinet will effectively limit the fault current to levels for use with bkrs. I prefer fuses - particularly those with rejection features - over bkrs simply because they are cheaper and act faster. Not everyone will agree with me but I think cheaper in this case is better. The choice is yours however.

 

[jbartos]

Suggestion to the previous posting: Please notice my earlier posting:
jbartos (Electrical) Dec 3, 2003
Suggestion: To feel safe around circuit breakers, it is necessary to calculate short circuit currents for the power distribution upstream and down to the circuit breaker bus. The calculated short circuit current must not exceed the circuit breaker short circuit current rating also known as interrupting rating or interrupting capacity. It is a good practice to have a reasonable design margin left, e.g. 15% or so.