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What earthing arrangement is this?
Former Community Member
The supply is from a private transformer in a four core cable 3ph + n. The cable armour is earthed and connected to the MET. However there is also a green and yellow cable connected to the neutral terminal at the main isolator going back to a the transformer casing. The transformer is only 5 or 6 metres away. I think this must have been intended to make it a tncs supply but seems to me to just create parallel neutral conductors. Or is it tn-s-c-s? I have only been able to go off visual inspection because I could not disconnect the supply..
It depends on the actual resistance of the HV and LV electrode, Kelly
Traditionally, if it was under 1 ohm, we combined them and called the site "cold" - usually the HV side was in underground cabling with a metallic sheath
If it wasn't we separated them by some distance so the LV electrode didn't pick up the HV rise of earth potential - we called that a "hot" site - usually supplied by an overhead HV system with no over running earth conductor
We now calculate the actual ROEP and base what's tolerable depending on expected speed of disconnection (keep in mind we often have intentional delay on the HV side to allow for discrimination) - unless you happen to be wandering around the transformer when wet and naked, it can easily get to somewhere near 2000V under your boots and still be considered "safe". Historically, most electrical equipment would be able to withstand a 2kV voltage being impressed on it - not so sure that equipment today is built to that kind of standard. Telephones are a particular problem, however - not that many still grip a handset of a traditional phone.
In practice, with high-speed protection, a site is classed as "hot" if the most onerous fault causes a ROEP at the substation boundary of 650v or more. With slower protection, a site is classed as "hot" if the most onerous fault causes a ROEP at the substation boundary of 430v or more. If neither is true, the site is cold
I agree that the cable connecting the system neutral bar to the case of the transformer sounds really suspect, however
It depends on the actual resistance of the HV and LV electrode, Kelly
Traditionally, if it was under 1 ohm, we combined them and called the site "cold" - usually the HV side was in underground cabling with a metallic sheath
If it wasn't we separated them by some distance so the LV electrode didn't pick up the HV rise of earth potential - we called that a "hot" site - usually supplied by an overhead HV system with no over running earth conductor
We now calculate the actual ROEP and base what's tolerable depending on expected speed of disconnection (keep in mind we often have intentional delay on the HV side to allow for discrimination) - unless you happen to be wandering around the transformer when wet and naked, it can easily get to somewhere near 2000V under your boots and still be considered "safe". Historically, most electrical equipment would be able to withstand a 2kV voltage being impressed on it - not so sure that equipment today is built to that kind of standard. Telephones are a particular problem, however - not that many still grip a handset of a traditional phone.
In practice, with high-speed protection, a site is classed as "hot" if the most onerous fault causes a ROEP at the substation boundary of 650v or more. With slower protection, a site is classed as "hot" if the most onerous fault causes a ROEP at the substation boundary of 430v or more. If neither is true, the site is cold
I agree that the cable connecting the system neutral bar to the case of the transformer sounds really suspect, however
