CPC size.

[johnboy6083]

hi gents. ive got a possible problem at work.

ive wired a supply from a switchfuse to a new VSD panel. the cable is a 4c 150mm2. from the VSD there is a 3c 150mm2 to a 60kW pump, standard induction motor. ive pulled in the cables as per the cable schedule, and the CPC is a 25mm2 single core earth. this strokes me as small.

Ive got home todayand tried the adiabatic to see if its big enough.

i found on a fuse manufactureres website that for a 60Kw motor, a 250A bs88 fuse is suitable. (it is to be fused).

ive estimated that the Zs of the motor circuit (20m) is going to be about 0.01 ohms. (the water site is fed by its own transformer). i will confirm when i get back on site on thurs.

this gives me a PFC of 39.836 Ka.
the operating time of a fuse with that fault current is 0.1 secs.
vaklue for k is 176.

i dont really understand the I2t of the protective device, so havent reduced the fault current with that in mind.im happy to be educated on this though.

when doing the adiabaicv gives me cable size of 95mm.

can any of you kind gents please verify this for me please, and point out any flaws in my calculation?

im going to speak to the designer about it, but would like some assistance from here too.

thanks,

john

 

[mechelec]

your Zs does seem optimisticly low. Your Ze at the cutout even next to the PT will be more likely higher than your 0.01 ohms. At most ground mounted substations the Earth loop impedence is around 0.07 ohms due to transformer internal impedence. your PFC is incredibly high 16KA is seen as the maximum possible in areas of dense distribution primary sub stations ie middle of london
If you guestimated a Zs of 0.10 that would give a fault current of 2300A (230v/0.10ohms) and a disconnection time of 0.5 seconds for a bs88 fuse and k value of 176 using the adiabatic equation would give you 9.24mm squared. I think.
This is from the information given by the OP

 

[johnboy6083]

thanks mechelec. i have measured similar sites befopre and recorded values of <0.01ohms, but have never tried the method that IQ uses (inserting an impedance in series with test leads, to improve accuracy). the square root only covers the top of the equation according to 543.1.3.

doing this with the square root only covering the top of the equation gives me a value of 4.132.

 

[mechelec]

Sorry I was edditing my reply as you posted yours. I had a bit of a senior moment about the square root for some reason. I would be interested to hear what you Ze and Zs are on site.

 

[johnboy6083]

right, i tested the Ze on site, and my fluke read, <0.01. i didnt have the time to try IQ's method, as i had other problems with a 400Aswitcfuse to contend with.


I must also add that wthe pump is a 160Kw pump, with a FLC in delta of 280A, a pf of 0.87, and i think this means efficiency, IE2-94.9%
i was researching this earlier, and in the electrical installation design guide , I managed to find the impedance for supply trannys. its on page 165, table F5.

ive had to do a little assumption, nut The singles supplying our site, are 630mm. tabulated rating for is 992A. im guessing a PF of 0.8, and so end up with a 800kVA trannyf
resitance per line is 0.00291, and reactance is 0.0107. the formula for total impedance is Z= square root(0.00291 squared + 0.0107 squared).

this gives me a line impedance of 0.011 ohms. the rest of the run, im gouing to guess that the resistance is negligble, as its only a 20m run from the tranny, and i cant find the resistance.

using this impedance of 0.0011ohms, i can get a more accurate figure for PEFC, which i work out to be, 36.3kA.

have i made any errors with my caklcs, and is this a viable way to calculate fault currents.

Thanks,

John

 

[bomjac]

Couple of points here. A few of them are OT to the OP but worthwile to know them anyway.

Generally speaking with a VSD driven motor, the overload protection of the motor and motor cables is taken care of by the VSD cabinet itself. Any faults that occur within the VSD itself are covered by the Semi conductor fuses that reside with the VSD itself. The purpose of the protection fuse in the feeder is to protect the VSD cabinet and feeder cable itself, nothing else. With a 280 amp FLC motor, I would say a 355 amp fuse would be sufficient for the feeder. If there is no internal fuse protection on the VSD then the fuse on the feeder to it must be of the type specified by the VSD manufacturer.

You mention a 25mm cpc. The VSD / drive unit I was looking at today (ABB ACS800 80kW) lists the CPC for the motor to be at least 50% of the line conductor size. For the feeder - VSD that may well be within your limits, but I still think that it is good working practice to keep the CPC at 50% line conductor size in this instance. Without knowing the make of your drive I would say that chances are it will be similar. Any VSD driven motor also has to be properly earthed to the motor due to the high line - earth voltages produced (as much as 1kV on a 440V system).

Best thing to do is download the makers manual for the proper values they are expecting.

 

[Knobhead]

Any faults that occur within the VSD itself are covered by the Semi conductor fuses that reside with the VSD itself. The purpose of the protection fuse in the feeder is to protect the VSD cabinet and feeder cable itself, nothing else.

Just to throw a little pebble in the water, this is one I’ve never understood. I installed a new DC thyristor panel to replace a early 60’s panel. I had to up rate the semiconductor fuses from 400A to700A just to get the bloody thing going! The starting and running loads were the same.
At least the new panel had 4-20mA I/O so I could control it from the PLC.