Split Phase Isolation

[BillBirch]

My DNO is about to upgrade my overhead supply from the existing transformer from 230V Single Phase (2 wire) TC-N-S (PME) to 230V / 460V split phase to accomodate a 20kW wind turbine. This means a 3 wire incomer. I have some issues over the positioning of the additional meter required but that's for another question. The supply will be provided by the same but now center tapped transformer with one "phase" going to the domestic CU and the other "phase" going to the G59 turbine enclosure and inverter bank. I will (and am obliged to) fit a lockable isolation switch - RECS2 say - to each live and neutral pair. However, the two neutrals have a common connection at the DNO junction box. Has anyone experienced or heard of a fault in the neutral connection that could result in 460V between either of the live feeds and the common earth ?? I have not come across any modern switch that differentially opens or closes to ensure that the live supply isolates first and powers on last i.e before and after neutral disconnect...............or am I worrying needlessly ??

 

[Andy™]

most isolators have neutral make first & break last

you can also get a REC4 isolator

unless you have a serious fault, you cannot get 460V to earth. only betwen phases

 

[BillBirch]

Thanks Andy, that's reassuring, although it isn't obvious from the specs ! I did consider a REC4 but I was under the impression that isolation has to be downstream of the meter(s) hence 2 x REC2. I am still concerned about the possibility and consequences of 460V between phases - that would make the toaster super efficient...........Are you aware of any MCB style device that can sense and trip quickly enough - the effect of a phase to phase fault would be similar to a power surge / lightening strike.

 

[Andy™]

there will always be 460V between the phases?

 

[BillBirch]

An update .......I finally got planning permission after 12 months of planning delays followed by 3 days of frantic activity. Last week (!) saw the concrete base poured and the DNO came to upgrade the line as described in my post of the 5th August 2011. The DNO would not accept responsibility for connecting the meter tails to my REC 4 isolator (4 pole) - as the neutral was switched. They believe that there is a real risk of the neutral breaking after one, or other, or both, L1 and L2 and consequently possibly imposing 460 volts on the system. They were quite happy to fit a REC 4 with a linked neutral i.e. not switchable. I now have a poly phase meter with L1 and neutral feeding the domestic CU and L2 and neutral feeding the turbine inverter bank. What is also quite interesting is that I now have 246V rather than the 238V I had before the rewire - must be the new 95mm overhead cable and the tight connections. I expect I will get a flood of blown lightbulbs now !