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OK, time to fess up.

I was responsible for this type of testing for a switchgear manufacturer in the early 90's, although this was at higher voltages and fault level.

Some failures were spectacular, unfortunately the consequences in some real life incidents were truly horrific.
 
Thankfully I have never had to work on anything that big, nor deal with the human results of that sort of failure, but I was rather disturbed by the results of testing at what was stated to be the manufacturer's ratings.

I see a lot about arc-flash risk mentioned in relation to USA wiring but not nearly as much over here. I had wondered if that came down to the UK commonly using HRC fuses as the fault protection for many big motors and DBs, instead of MCCB and the like?
 
Thankfully I have never had to work on anything that big, nor deal with the human results of that sort of failure, but I was rather disturbed by the results of testing at what was stated to be the manufacturer's ratings.

I see a lot about arc-flash risk mentioned in relation to USA wiring but not nearly as much over here. I had wondered if that came down to the UK commonly using HRC fuses as the fault protection for many big motors and DBs, instead of MCCB and the like?
Although a lover of fuses, both for the great current limiting effect, but also ease of use, I do have some suspicions about that video.

63/70kA is an unusual rating for a mccb, that sounds like the peak asymmetrical rather than normal rating, and in addition many mccb have one rating for "normal use" - where the device is suitable for continental use, and a higher "clearing" rating where the mccb will clear the fault, but no longer suitable for use again.

In this latter case gasses and debris may escape the breaker, but be contained within the enclosure, noting the enclosures have been omitted in this video.

If the tests were in line with this latter case, most of the tests would have passed, although some would most certainly not!
 
There was another comparison video where they had a MCCB tested at 50% of its rating repeatedly and only lasted something like 8 faults, which makes some sense.

One might like to think high PFC faults like that are very rare, so several resets is a lifetime for the system, but I guess there will be some applications when that is not the case.
 
There was another comparison video where they had a MCCB tested at 50% of its rating repeatedly and only lasted something like 8 faults, which makes some sense.

One might like to think high PFC faults like that are very rare, so several resets is a lifetime for the system, but I guess there will be some applications when that is not the case.
Yeah, I think the expectation is a bit too much tbh.

Most people do not really understand just how much energy is contained in a high current fault, even at low voltage.

A fuse cuts off the current at a respectable level, an acb/mccb etc, doesn't really do much in this regard, so the magnetic forces in cables etc become huge, certainly whenever I have needed to calculate it, it almost always shows the cable would burst.

Yet I have still come across instances on industrial sites where the cables are just too small, yes sufficient size for load current, fault impedance, and even i^2t in some cases, but would split apart due to the fault level!

However, the use of mcbs/mccbs is "modern" and much better than those "old fashioned " fuses it would appear.
 
As you say, the MCB/MCCB is seen as modern and fuses as old, but without a great deal of thought or reasoned arguments one way or another.

For domestic CU fuses are not good, more so the rewirable sort, due to the unskilled nature of most folk needing to reset them. But once you get in to industrial situations that becomes less of an argument and certainly if there is any danger of them being closed on to a high energy fault I would be very unhappy about that being done by anyone not skilled and PPE'd-up to deal with it going wrong.

Sadly there seems not to be an option for small fuse/switch units compatible with the MCBs/MCCBs for DB in those cases when you would be better off with the protection let-through, selectivity when cascaded, etc., of a fuse.

As an aside, and since you have actually been involved in the testing, how do those tests generate the PFC? I can't imagine the grid would be happy with a 5MW or whatever sub-station being shorted out for testing, so do they have flywheels and motor/generator sets or something to dump the fault energy as needed?
 
As you say, the MCB/MCCB is seen as modern and fuses as old, but without a great deal of thought or reasoned arguments one way or another.

For domestic CU fuses are not good, more so the rewirable sort, due to the unskilled nature of most folk needing to reset them. But once you get in to industrial situations that becomes less of an argument and certainly if there is any danger of them being closed on to a high energy fault I would be very unhappy about that being done by anyone not skilled and PPE'd-up to deal with it going wrong.

Sadly there seems not to be an option for small fuse/switch units compatible with the MCBs/MCCBs for DB in those cases when you would be better off with the protection let-through, selectivity when cascaded, etc., of a fuse.

As an aside, and since you have actually been involved in the testing, how do those tests generate the PFC? I can't imagine the grid would be happy with a 5MW or whatever sub-station being shorted out for testing, so do they have flywheels and motor/generator sets or something to dump the fault energy as needed?
It's a generator with a large flywheel, you run the machine up to speed, then set the sequence switch in operation. It starts the recording systems, disconnects the motor supply, then closes the generator output through reactors and resistors (to get the correct fault level and x/r ratio etc) , if needed it then sends the trip signal to the switch under test (for opening tests only), then after the required time, opens the circuit again (assuming the breaker under test didn't clear the fault), then stops recording etc.

The generator, reactors, capacitors, resistors, make switch and interruptors are all specially designed for this use.

The system at Trafford Park could test up to 2000MVA as a straight fault, above this we had to use "synthetic" testing (one phase at a time with seperate application of current and voltage) The one at Brush switchgear was slightly bigger at 11kV - at other voltages we had more capacity.

The main test station at KEMA, was huge in comparison to us, unfortunately I only went there as a customer, never got to operate, it was much more modern, electronic timing mechanisms rather than the rotary sequence assembly we had!

(A synchronous motor spun the barrel, and you moved the buttons around it such that they hit the microswitches in the correct sequence and timing to ensure the fault arrived [or the switch under test opened] on the peak, or whatever part of the cycle it needed as per test!)
 

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