I think this has been covered once before, but one of the current IET publications shows on the schedule of test results Max Zs as 1666 when protected by RCD (possibly RCBO as well) even when having a TN-C-S connection. I wondered what was the current opinion on this.
You got me. Which IET publication is this? There is nothing in the model forms for max Zs, though of course some generic forms have this in the circuit schedules as a prompt for the Installer. I would have thought any figure would be to achieve ADS, in the first instance. If an RCD was required to achieve ADS then it would be the higher figure[ for the RCD ]. If the fuse/mcb could achieve it, then the max Zs is for that, regardless if there is an RCD or not. Simply, for TN it would not be the RCD figure as it suggests something is wrong with the circuit.
Now I know some will disagree with the last sentence, as there seems to be some Schemes suggesting that an RCD can be claimed for ADS on a TN circuit that is a bit long and the cables a bit small, resulting in an overly large earth fault loop path, in order to save money on a properly sized set of conductors ?.
Now it could hinge really on what you are verifying for what purposes. So if it was for initial verification in a TN , it would be the fuse, but , perhaps if an in-service verification [EICR], it could be the RCD if the original circuit is so rough that is all there is for ADS purposes ?. RCDs are for fault protection and additional [ supplementary ] protection, now there is carelessness of the "user" , which is fine, but there does seem to be some mission creep where it seems to be for the carelessness of the installer also.
You got me. Which IET publication is this? There is nothing in the model forms for max Zs, though of course some generic forms have this in the circuit schedules as a prompt for the Installer. I would have thought any figure would be to achieve ADS, in the first instance. If an RCD was required to achieve ADS then it would be the higher figure[ for the RCD ]. If the fuse/mcb could achieve it, then the max Zs is for that, regardless if there is an RCD or not. Simply, for TN it would not be the RCD figure as it suggests something is wrong with the circuit.
Now I know some will disagree with the last sentence, as there seems to be some Schemes suggesting that an RCD can be claimed for ADS on a TN circuit that is a bit long and the cables a bit small, resulting in an overly large earth fault loop path, in order to save money on a properly sized set of conductors ?.
Now it could hinge really on what you are verifying for what purposes. So if it was for initial verification in a TN , it would be the fuse, but , perhaps if an in-service verification [EICR], it could be the RCD if the original circuit is so rough that is all there is for ADS purposes ?. RCDs are for fault protection and additional [ supplementary ] protection, now there is carelessness of the "user" , which is fine, but there does seem to be some mission creep where it seems to be for the carelessness of the installer also.
