would someone please explain the reasons for having a 0.4 second disconnection time and a 5 second disconnection time, why is there a big difference between the two readings
disconnection times
- Thread starter vertigodj
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Very basically
0.4 are things you would be more likely holding or touching so more chance of a shock.
5secs are things not so likely to go wrong or be touched. so less likely to give you a shock
0.4 are things you would be more likely holding or touching so more chance of a shock.
5secs are things not so likely to go wrong or be touched. so less likely to give you a shock
The 0.4 sec. time is based on the work of Biegelmeier and Lee who did work on the effects of current through the human body. There is a graph towards the back of GN8.
The 5 second time is bit of a mystery to which no two people could give you a straight answer. It is roughly based on what could be a reasonable disconnection time for a circuit which is highly unlikely to have anyone in contact with it eg a distribution circuit
The 5 second time is bit of a mystery to which no two people could give you a straight answer. It is roughly based on what could be a reasonable disconnection time for a circuit which is highly unlikely to have anyone in contact with it eg a distribution circuit
The legend involves a committee member who has a vested interest in not having the circuits in his employers factory be deemed none compliant, a quick fag packet calculation was carried out and he reckoned 5 seconds would not cause him any difficulty
No idea if its true, but it wouldn't supprise me, things like that make the world go round!
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The legend involves a committee member who has a vested interest in not having the circuits in his employers factory be deemed none compliant, a quick fag packet calculation was carried out and he reckoned 5 seconds would not cause him any difficulty
No idea if its true, but it wouldn't supprise me, things like that make the world go round!
IET, Mr Phoenix ?
hi
the 5 sec disc. is a spillover from the 14th edition....in this book they talked of close and coarse protection, and fusing factors.
example....
it takes around 2x the rated current to blow a rewireable fuse so they set the blow at 3x in the old book.it was 3x because it was classed as "coarse protection" this "3" was called "fusing factor"
so for a 30A fuse, 3x30 is 90A. i havent got the regs to hand but it should be around 5 seconds....
i think the fusing factors were for a cartridge unit was 2.4 and an mcb 1.5, but it was a long time ago
the 5 sec disc. is a spillover from the 14th edition....in this book they talked of close and coarse protection, and fusing factors.
example....
it takes around 2x the rated current to blow a rewireable fuse so they set the blow at 3x in the old book.it was 3x because it was classed as "coarse protection" this "3" was called "fusing factor"
so for a 30A fuse, 3x30 is 90A. i havent got the regs to hand but it should be around 5 seconds....
i think the fusing factors were for a cartridge unit was 2.4 and an mcb 1.5, but it was a long time ago
Rapid disconnection ( 0.4 seconds or less ) if current is leaking out of the protected circuit as the leakage could be causing immediate danger to life ( shock ) or fire ( overheating of material not intended to carry current ). This protection by an RCD
Slow disconnection when the current is contained within the circuit and is higher than the current carrying capacity of the cable but not high enough to cause immediate danger of fire from overheating to ignition point of the cable or surrounding materials. But disconnection must occur before the thermal mass of any part of the cable can reach ignition temperature. This protection provided by an MCB
Slow disconnection when the current is contained within the circuit and is higher than the current carrying capacity of the cable but not high enough to cause immediate danger of fire from overheating to ignition point of the cable or surrounding materials. But disconnection must occur before the thermal mass of any part of the cable can reach ignition temperature. This protection provided by an MCB
G
Goldberg
Would that be a 400ms RCD?
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A different world!
Excess current protection, close (definition): ECP which will operate within 4 hours at 1.5 times the designed load current of the circuit which it protects.
Devices affording close ECP include BS88's with a fusing factor no greater than 1.5 and MCB's and MCCB's to BS3871.
Excess current protection, coarse (definition): ECP which will not operate within 4 hours at 1.5 times the designed load current of the circuit which it protects.
Devices affording coarse ECP include BS 88's other than those listed above and BS 3036's.
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