S
scotsparky
I know this a fairly gray area but can anyone explain how to prove OCPD diversity with HRC fuses and MCBs?????
use the fuse characteristic charts in the back of the regs..
compare the lines..
compare the lines..
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scotsparky
hio col is that true even if it different tyes of OCPD. I know its a grey area on my C&G course it was mentioned and the lecturer asked if we all had an extra day to go through it he also said there can be differences in manufactures between the Pre arc and Let through current.
to be honest I was just taking a shot in the dark...
but a 100A type B breaker will trip in 0.1 - 5 seconds at 500A, whereas a BS88 100A fuse will blow in 5 seconds at 550A..
but a 100A type B breaker will trip in 0.1 - 5 seconds at 500A, whereas a BS88 100A fuse will blow in 5 seconds at 550A..
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scotsparky
Adam-151 yes mate thats it sorry brain fart when typing
Basically, if the downstream device's energy let through I²t is smaller than the pre-arc energy of the upstream device they should discriminate. Some manufacturers do charts telling you which devices fully discriminate - it is easier with fuses than MCBs as MCBs tend to have a higher energy let through owing to them being mechanical and therefore take a minimum time to react.
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scotsparky
sparky123 where can i get information about it or is it all manufacturers info?
Some info on GEC Redspot on page 13-14 (and on others if you scoll down) here: http://www.gepowercontrols.com/eu/r...ads/DS_Catalogue_fuses_redspot_english_uk.pdf
Its a complicated subject, yes, but quickly... here goes...
Protective devices have a figure called let through energy, which basically specifies how much energy goes into a fault before the device opens, its specified in Amp squared seconds (or KA squared secs) and refered to as the I²t value , which while not a unit of energy, it is directly prortional to the energy involved. Its simply the product of the fault current sqaured multiplied be the time the device takes to open... so say I have a fault level of 500A and my fuse takes 0.2 seconds to disconnect, then thats 500x500x0.2 = 50,000
Fuses also have a second value called the pre-arcing, this is because when the fsewire has melted the current flow doesn't stop immeditaly, the arc has to clear first. Basically when co-ordinting two devices, the I square t value of the downstream one has to be smaller than the I square t of the upstream one, if if fuses, the total value of the downstream one has to be smaller than the pre-arcing of the upstream one (cos the fault needs to be disconnected before the wire has vapourised on the upstream device!)
Sounds simple, but the I²t of a device is not a fixed value, for example in the case of MCBs once the current is high enough to disconnect the device, it'll disconnect the device in a reasonaly quick time (somewhere upto 100ms), but this time will be roughly constant [mechanical device, the speed at which it can operate is limited] however much the fault current is... whether its half, one or two KA, so the I²t value will increase in protion to the square of fault current.... fuses on the other hand, the value will decrease as fault current increases towards an ideal value (the energy to vaporise a fusewire is a set value, but if you do it slowly, the energy disapates as you are doing it, and you need more to do it to replace this... imagine heating a kettle on a candle as opposed to a hob)
Can help to plot them on a graph and see if you acheieve discrimination for the expected fault levels
Sure I've forgotton something, so if I have failed in confusing you more, gimme a shout
Protective devices have a figure called let through energy, which basically specifies how much energy goes into a fault before the device opens, its specified in Amp squared seconds (or KA squared secs) and refered to as the I²t value , which while not a unit of energy, it is directly prortional to the energy involved. Its simply the product of the fault current sqaured multiplied be the time the device takes to open... so say I have a fault level of 500A and my fuse takes 0.2 seconds to disconnect, then thats 500x500x0.2 = 50,000
Fuses also have a second value called the pre-arcing, this is because when the fsewire has melted the current flow doesn't stop immeditaly, the arc has to clear first. Basically when co-ordinting two devices, the I square t value of the downstream one has to be smaller than the I square t of the upstream one, if if fuses, the total value of the downstream one has to be smaller than the pre-arcing of the upstream one (cos the fault needs to be disconnected before the wire has vapourised on the upstream device!)
Sounds simple, but the I²t of a device is not a fixed value, for example in the case of MCBs once the current is high enough to disconnect the device, it'll disconnect the device in a reasonaly quick time (somewhere upto 100ms), but this time will be roughly constant [mechanical device, the speed at which it can operate is limited] however much the fault current is... whether its half, one or two KA, so the I²t value will increase in protion to the square of fault current.... fuses on the other hand, the value will decrease as fault current increases towards an ideal value (the energy to vaporise a fusewire is a set value, but if you do it slowly, the energy disapates as you are doing it, and you need more to do it to replace this... imagine heating a kettle on a candle as opposed to a hob)
Can help to plot them on a graph and see if you acheieve discrimination for the expected fault levels
Sure I've forgotton something, so if I have failed in confusing you more, gimme a shout
There's some good reading here:
http://www.voltimum.co.uk/news/6114....circuitbreakers/Guide-to-discrimination.html
*edit* and also a goodly bit on the subject here starting page 6.
- just in case you're finding it hard to sleep.... http://help.tradingdepot.co.uk/electrical/eaton-mem/the-guide-cat865-circuit-protection control.pdf
http://www.voltimum.co.uk/news/6114....circuitbreakers/Guide-to-discrimination.html
*edit* and also a goodly bit on the subject here starting page 6.
- just in case you're finding it hard to sleep.... http://help.tradingdepot.co.uk/electrical/eaton-mem/the-guide-cat865-circuit-protection control.pdf
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