Hi All,
Just been writing something for someone, and I thought I would post some of what I wrote here for constructive criticism from you lot!!!
One thing that causes no end of argument is the maximum Zs for a circuit with an rcd [i am assuming for the sake of argument a 30mA one here]....
At a first read, according to the regs, you have a CHOICE of device to achieve ADS, an OCPD or alternatively, an RCD, and, if you choose an RCD, then the regs VERY DEFINITELY say that the max Zs is 1667.
It is all in the table 41.5 [i only got the old red book] It is all there as clear as day....
BUT, wait a minute.... Look at the table before that... table 41.3........ Each heading reads for example, the first one; "Type "B" circuit-breakers to BSEN 60898 AND the overcurrent characteristics of RCBO's to BSEN 61009-1"
Why would they even care about the Zs to operate the overcurrent part of an RCBO if the RCD half of it will do the job for us??
The answer, it seems to me, is this,
SOME RCCB devices rely on the supply voltage to operate. RCBO's do for a start off, which is why i presume, they have included them in the table i refer to above...
Now have a look at this link. See how the RCD's are described as "line voltage independant" BUT the RCBO's as "line voltage dependant" VERY interesting eh!!
http://www.eaton.com/SEAsia/ProductsSolutions/Electrical/ProductsServices/PowerDistributionComponents/MiniatureCircuitBreakersProtectionAccessories/DINCircuitProtection/RCBO/index.htm
Now think of this scenario; You have a direct short L/E This will try to drag the supply voltage right down and, if the fault is close to the cutout, [so PEFC will be very high] might well do so. Now, in a TNCS system, the earth is connected to neutral at the cutout, they are one and the same as far as the "other" side of the cutout is concerned... Now, assuming the supply conductors have the same impedance, the voltage will be divided across our fault. The phase conductor voltage will be dragged down, and the voltage on our earth will be dragged up to meet it. As the earth is connected to the neutral, it will try to drag the neutral voltage up. The result can or could be, that there is not enough L/N voltage left to operate the thing.....
My verdict???
If you use an rcd, [if you NEED to and have no choice] use the 1667 figure, not literally of course, but you could take advantage of the increased latitude the use of the RCD will allow. If you use an RCBO use the figures in table 41.3 which are also the figures for mcb's... That seems best to me.
Constructive criticism please........
[SIZE=12pt]john...[/SIZE]
Just been writing something for someone, and I thought I would post some of what I wrote here for constructive criticism from you lot!!!
One thing that causes no end of argument is the maximum Zs for a circuit with an rcd [i am assuming for the sake of argument a 30mA one here]....
At a first read, according to the regs, you have a CHOICE of device to achieve ADS, an OCPD or alternatively, an RCD, and, if you choose an RCD, then the regs VERY DEFINITELY say that the max Zs is 1667.
It is all in the table 41.5 [i only got the old red book] It is all there as clear as day....
BUT, wait a minute.... Look at the table before that... table 41.3........ Each heading reads for example, the first one; "Type "B" circuit-breakers to BSEN 60898 AND the overcurrent characteristics of RCBO's to BSEN 61009-1"
Why would they even care about the Zs to operate the overcurrent part of an RCBO if the RCD half of it will do the job for us??
The answer, it seems to me, is this,
SOME RCCB devices rely on the supply voltage to operate. RCBO's do for a start off, which is why i presume, they have included them in the table i refer to above...
Now have a look at this link. See how the RCD's are described as "line voltage independant" BUT the RCBO's as "line voltage dependant" VERY interesting eh!!
http://www.eaton.com/SEAsia/ProductsSolutions/Electrical/ProductsServices/PowerDistributionComponents/MiniatureCircuitBreakersProtectionAccessories/DINCircuitProtection/RCBO/index.htm
Now think of this scenario; You have a direct short L/E This will try to drag the supply voltage right down and, if the fault is close to the cutout, [so PEFC will be very high] might well do so. Now, in a TNCS system, the earth is connected to neutral at the cutout, they are one and the same as far as the "other" side of the cutout is concerned... Now, assuming the supply conductors have the same impedance, the voltage will be divided across our fault. The phase conductor voltage will be dragged down, and the voltage on our earth will be dragged up to meet it. As the earth is connected to the neutral, it will try to drag the neutral voltage up. The result can or could be, that there is not enough L/N voltage left to operate the thing.....
My verdict???
If you use an rcd, [if you NEED to and have no choice] use the 1667 figure, not literally of course, but you could take advantage of the increased latitude the use of the RCD will allow. If you use an RCBO use the figures in table 41.3 which are also the figures for mcb's... That seems best to me.
Constructive criticism please........
[SIZE=12pt]john...[/SIZE]