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Posted By Topic: Why Double pole overcurrent for isolated supplys?

Rhino
Jan 23 2018 22:46

Why do "isolating" or "floating" supplies require double pole protection ?
Often an exception in Extra low voltage however generally required in Low voltage situations.. eg solar panels, isolated inverters, generators with isolated supplies etc

In an isolated transformer, i assume that a fault to earth would be undetected, however this would just mimic a normal earth referenced supply and once someone came in contact with the unreferenced conductor(say neutral ) the circuit breaker( single pole in phase conductor) would operate and isolate the earth fault, the neutral would still have 230V to earth however coming into contact with this shouldn't be a problem as the earth fault would have been isolated from the point of supply? Any others reasons?

   

pluto
Jan 24 2018 06:49

Rhino Jan 23 2018 22:46
Your question
Why do "isolating" or "floating" supplies require double pole protection ?
Often an exception in Extra low voltage however generally required in Low voltage situations.. eg solar panels, isolated inverters, generators with isolated supplies etc

My reply
In the Wiring Rules (based on the IEC rules) there is a blanket statement which says that all "unearthed conductors are required to be switched and be able to be isolated". This means in the case of an single phase isolated supply that switch poles must be in both conductors as none are earthed in normal operation.

The prime reason being that the cables could be operating at voltages of greater than 50 volts to the mass of earth which is lower limit of the shock voltage.

Then a secondary statement which says that the neutral conductor in a TN-C-S (MEN) supply is reliably earthed the provision of isolating switch pole can be omitted. This is because the neutral conductor (and the load and phase cable) is most unlikely to be at greater than 50 volts to the mass of earth.

In the case of ELV the requirement for isolating switch drops back to a only one pole switch because all of the circuit is most unlikely to be at greater than 50 volts to the mass of earth.




   

pluto
Jan 24 2018 11:56

My comments above apply to isolation they also apply to over current devices for the same reasons.
   

Rhino
Feb 08 2018 21:31

Figure 4.3 ASNZS 3010, 2017 details an isolated supply with a plug in appliance inlet at the fixed installation.
The neutral is switched at the change-over device, therefore you have an isolated supply right up to the changeover switch, when mains supply is active there is no neutral to earth connection on the generator side.

Why is the generator supply switch a single pole device and not double pole?.. as all unearthed conductors need to be switched?
   

AlecK
Feb 08 2018 22:59

When c/o is switched to "generator", there is a connection between Generator N & installation E.
When c/o switched to "normal", there is no circuit for the genset output.

So the system as a whole is never running as an isolated supply.

The appliance inlet N has to be switched because otherwise the N pin could be an accessible live part
   

Rhino
Feb 10 2018 19:04

Yes I understand the installation /system wouldnt run as an isolated supply , however the part of the system from generator to changeover would be running as an isolated supply , and under fault conditions inbetween single pole switch and c/o switch a fault could go unnoticed and the single pole switch would inadequately isolate the c/o switch. ?
   

pluto
Feb 10 2018 20:36

Rhino Feb 10 2018 19:04
Your comment 1
Yes I understand the installation /system wouldnt run as an isolated supply , however the part of the system from generator to changeover would be running as an isolated supply , and under fault conditions inbetween single pole switch and c/o switch a fault could go unnoticed and the single pole switch would inadequately isolate the c/o switch. ?

My comment
Assumming the generating set is plugged into the main switchboard appliance inlet the flexible cord to the generating set is also plugged in to the gnerating set and the changeover device is in the generator position you find that the neutral bar is connected to the generating set neutral output and hence is earthed by the MEN link in the switchboard. This means the generating set output connections now has am earth reference by the neutral connection and works like a normal MEN connection.

May I suggest you draw the circuit down on a peice of paper twice and put the changeover device in two conditions, mains and generator positions and then trace all connections to show that the above notes are correct.