You can get substantial currents in a N→E fault. Not necessarily from the circuit involved. The SP RCBO will detect but not disconnect the fault whereas SPSN or DP will.
Under no circumstances should SP RCBO’s alone be used on a TT supply. I know it’s not the case here, I’m making the point. How many times do we get threads on here “I’m changing a CU on a TT supply”? Members blithely come back with “fit RCBO’s”. The worse advice that could be given unless the type is specified.
Tel, you of all people should appreciate the problem.
Being lazy here...
Assuming negligible conductor impedances, am I right in saying that half of the installation current would be flowing through the faulty circuit's cpc and off down the supply Earth?
In the ideal world with as you say negligible impedance then yes, half the current would take the N→E rout. I don’t know about a 1mm CPC, it would become a fuse.
I know you’ve read the article about transformers elsewhere. I explained why impedance is introduced in to the star point of MV transformers. The N→E fault level could be of such magnitude as to melt the cable lead sheath and armourings.
You can get substantial currents in a N→E fault. Not necessarily from the circuit involved. The SP RCBO will detect but not disconnect the fault whereas SPSN or DP will.
Under no circumstances should SP RCBO’s alone be used on a TT supply. I know it’s not the case here, I’m making the point. How many times do we get threads on here “I’m changing a CU on a TT supply”? Members blithely come back with “fit RCBO’s”. The worse advice that could be given unless the type is specified.
Tel, you of all people should appreciate the problem.
i do, up to a point, but was just pointing out that a SP RCBO will do it's job in the majority of cases by dissing the L and removing the immediate danger when used for additional protection. fault protection is a different kettle of fish, especially in industrial installations.
i do, up to a point, but was just pointing out that a SP RCBO will do it's job in the majority of cases by dissing the L and removing the immediate danger when used for additional protection. fault protection is a different kettle of fish, especially in industrial installations.
You’re the domesticated lads, what protection would you normally use and what are the consequences of not protecting a circuit? The only way I can think of is DP MCB’s. They will trip given a N→E fault of the magnitude that will cause damage to cables. For some reason I can’t see you all rushing down that rout. There will be the usual cry of “it’s too expensive!” which is the usual cry with DP RCBO’s.
It all comes down to “how much are you willing to pay for safety?”
Not many circuits on an industrial site will be RCD protected. At the foundry I can only think of nine bits of kit on the two sections I looked after that was portable and therefore needing RCD protection. Not many considering the size of the plant. Anything else that was portable was 110V from fixed transformers.
The UK system is predicated on safety being disconnection of the line conductor, this obviously removes the shock hazard of the presence of a significant potential difference.
The hazard of having an undetected N-E fault (which I would expect to be present in about 50% of old domestic installs) is the ccc of the cables through which the fault current runs, the integrity of the connections of those cables (since earths generally tend to be ignored!) and the risk of sparking giving rise to fire.
In an otherwise well constructed installation an NE fault would split the current via N and E, since this halves the current in any conductor this helps avoid overcurrent problems, but the chance of the current from high powered circuits flowing through the cpc of say a lighting circuit brings the risk back up.
If all the connections are OK then there should be fewer problems with terminations than ccc of cables, particularly on earlier joints that tend to have a much larger terminal than strictly necessary for the current.
Again a good connection of all cables lowers the risk of arcing at faulty joints, though it is is often the main worry in thatched properties.
In general I am happy that single pole disconnection does not cause undue hazard, but in more modern installs the presence of RCD protection makes the nuisance value of unexpected tripping much higher because the single pole RCBO is not clearing the fault, the fault will be shifted to the next upstream device and cause unnecessary disconnection of non faulty circuits.
Single pole RCBOs may significantly reduce the fault current since the disconnected circuit is not generating a fault current and the fault point will be the highest resistance in the current path for the stray current so limiting the current flow via the fault.
I work in industrial commercial installations. The only double pole RCBO,s and DP+MCBs configurations we install are EX or agricultural installs
Ex because of the hazardous area
Agricultural DP rcbos are install for discrimination of the back up 300ma rcd
In industrial it generally is only the socket outlets that require rcd protection. So as I said any other N-E fault would go undetected. Although sp rcbos would highlight the N-E fault by not resetting( although as others have stated the fault would still be in circuit )
I work in industrial commercial installations. The only double pole RCBO,s and DP+MCBs configurations we install are EX or agricultural installs
Ex because of the hazardous area
Agricultural DP rcbos are install for discrimination of the back up 300ma rcd
In industrial it generally is only the socket outlets that require rcd protection. So as I said any other N-E fault would go undetected. Although sp rcbos would highlight the N-E fault by not resetting( although as others have stated the fault would still be in circuit )
Reply to EICR in a commercial premises, no RCD protection in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net
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