Isolation of TN supplies.

[nicebutdim]

I haven't seen a lot of discussion on this subject and have questions pertaining to isolation where TN earthing is present and any peculiarites which may exist between TN-S and TNC-S.

I see mostly 3 pole main switches on TN earthing systems, but do see quite a few instances where all live conductors are switched.

Furthermore, where loads are isolated what consideration should be given to this? I'm asking specifically in relation to the post linked below. It seems to be that the only time I see a 3P rotary isolator is on motor circuits, with 3P+N isolation being used in pretty much every other situation.

There's a gap in my learning here and it's one I'd like to resolve.

Low insulation resistance reading

4 pole main switches are fairly common here, but the point was more about the fact we're free to use them, whereas I believe they can not be used in IE. I wouldn't say free to use, you would only switch the neutral when absolutely necessary. Switching the neutral introduces a greater risk of...

www.electriciansforums.net

 

[MDJ]

I think switching 3 phase and neutral at a rotary isolator controlling an outside air con unit for example is perfectly normal, but fitting a 4 pole isolator or main switch in a DB which is 3 phase but has single phase circuits a no, no, I have personally seen a neutral break on a 3 phase DB which was controlled via a contactor and had a stop circuit controlling the coil and when the circuit energised so the board would become live the neutral hung and 400 volts went across all circuits in the board and tens of thousands of pounds of connected equipment burned out, when I was an apprentice it was not the norm to switch a neutral in a 3 phase system with single phase circuits connected so that is how I was taught and learned. I however love to hear other sparks opinions and thank the OP for staring this thread.

 

[nicebutdim]

This is certainly a broader conversation than I'd anticipated.

I've never been involved in wiring any transfer switches for generators, but have been involved with a few fairly sizeable manual UPS changeover switches. The auto side of things I know little about as it was handled within the UPS, but from memory the main changeover switched all 4 poles and the UPS output side did the same, but I think that neutral was monitored. On the other side I'm fairly certain the rectifier and bypass isolators switched only the three lines and had links botled through neutral.


Edit: We can probably all agree that where functional switching is concerned it would generally be a bad idea to switch neutral.

 

[pc1966]

I think larger & good quality UPS link the output N & E when on-battery, so they would need 4P switching as input N and output N cannot be assumed to be the same. However, I guess the batteries and inverter are always referenced to the output side N so only need the L switched to safely isolate for any maintenance work.

The ones I have used in the 5kVA region usually have both + & - of the battery pack on a connector so you isolate both before withdrawing the tray of batteries, then they are all isolated from the metalwork and a bit less dangerous to handle. I still don't like handling them!

 

[nicebutdim]

I had been thinking about these after you mentioned generator transfer switches and posted from memory, but since found an image which confirms those memories.

Not sure what size each UPS was. Cable size I think was 70 or 95mm2.

 

[pc1966]

Not sure what size each UPS was. Cable size I think was 70 or 95mm2.

Sound like something in the 100kVA range.

 

[westward10]

There is not quite the same thing as "reverse polarity" as you can have:

• L-L swap leading to wrong rotation. This is not obvious without specific test (closer to SP reversal) but will likely trouble machines using 3P motors as fan direction reversed, etc, or
• N-L swap and then almost certainly see SP loads on the two other L-(L instead of N) voltages burning out!

The first could go undetected if you only have SP loads, but the 2nd is likely to be found out pretty quickly so not likely to be discovered on EICR, etc!

Not really sure how this is overly relevant to what I suggested.

 

[timhoward]

Yep, although I often see 5 cores used on motor circuits as the was on hand and neutral ends up parked in a connector.

In my first sparking era (late 90's) I was always told to run a neutral to the isolator, as the chances are whoever is using the machine will want a socket and a work light next week, especially in the case of lathes.
But if a socket was added and the N came into play we certainly didn't swap to 4 pole isolators!
I'd imagine RCD requirements prevent this arrangement these days anyway.

The only real benefit to TP+N isolation seems to be easier testing. At least I think it's easier to know that everything upstream is isolated and you aren't finding upstream N-E issues, especially when going through the IR permutations.