Three phase and IBM

All, On 21 March 2011 03:33, Tony Duell <ard at p850ug1.demon.co.uk> wrote: [3 phase connections and RCDs] Sorry. Should have replied to that thread but the thread went sideways. I am guessing the 100A phase to ground breaker that was being claimed and the RCD/ELCB/GFI 30mA breaker are two different safety systems designed to detect and break current for two different purposes. The transmission network (at least in Australia where I have experience with the transmission infrastructure) have a requirement to break the current on a phase to ground/earth fault. This is to protect primary plant and system stability more than than people coming in contact with the conductors. At 66kV and above on lines designed to handle 10's to 100's of MVA, people are generally considered as self clearing faults. I seem to recall what was quoted was likely to be more of the distribution system, which, at the current specified would be to protect the local distribution transformer against a phase to ground/earth or a phase to phase fault. It may also be the case that someone recalculated the numbers and realised that the circuit breakers may not be rated to break the fault current. Note that the current that the circuit breaker is required to break when a fault is present can be significantly larger than normal operating current. Without seeing the wiring diagram of the installation and the associated calculations, really it is all guesswork. Although, I too would be a little concerned if some equipment (as opposed to large plant) was leaking close to 100A of current to earth. That sort of fault current to a low impedance earth would result in something getting hot! The RCD/ELCB/GFI are designed to break the current before it has a change to interrupt the heartbeat, if someone forms a circuit between a suitable active conductor and earth. This is a much smaller current designed to protect installations where people are much more likely to come into contact with the current carrying conductors. Note that for the pedantic. I have tried to use ground and earth as appropriate. Where I am using ground/earth, it is where I would be expecting a three wire transmission system with the center of the Y winding connected to a large metal pole driven into the physical earth near the transformer. [chomp] Simon -- ------------------------------------------------------------------------ "Well, an engineer is not concerned with the truth; that is left to philosophers and theologians: the prime concern of an engineer is the utility of the final product." Lectures on the Electrical Properties of Materials, L.Solymar, D.Walsh