Safety of isolated inverter and associated issues - help? thanks

[123MB]

Hi Guys,

I have been struggling with the design of a control cabinet supplied by an isolated inverter so unfortunately I have decided to bother you all!
This appears to be a pretty simple setup... so here goes
The system is supplied from an equipotentially bonded isolated inverter. The inverter has 12VDC input and on the output side of the inverter is a centre tapped secondary with 230VAC L-N and 120VAC L-G (the centre tap is grounded). I am not sure what is happening inside the inverter before the output transformer but it is likely a switching rectifier.
The inverter supplies a control cabinet (class I), which is dedicated to supplying a large baseball scoreboard (class I). The scoreboard is connected L-N (230VAC) and the earth connection on the inverter is connected to the equipotential bonding system which includes extraneous conductive parts of the control cabinet and the scoreboard.
The inverter output is isolated from the DC input and the inverter output has no neutral-earth link. So, theoretically the first insulation fault will not operate any protective devices. Despite this, our local standards prescribe that a residual current device is installed downstream of the inverter - but this RCD will never trip under normal operation.
So, I am confused about the safest way to arrange this setup.
1) What is the reason for the centre tapped transformer in these arrangements? The output is supposed to be floating, but is it really floating? I am confused…
2) If there is an insulation fault, will the centre tap act as a return path for fault current?
3) What is safer in your opinion? An isolated inverter or an inverter with a neutral-earth link and an RCD?
4) Considering the centre tap on the output of the inverter I assume that I will not be able to install a neutral-earth link as it would short one side of the secondary winding? Agree?
5) Generally, with floating supplies at or around 240VAC, can someone explain clearly to me where the danger is in terms of the first fault and the second fault – in terms of the return path for fault current?
Geez this is a bit long. Hopefully someone reads this! Thanks if you got this far

Michael

 

[sibeen]

So, theoretically the first insulation fault will not operate any protective devices.

OK, maybe I'm confused. If there is a short to earth in the control cabinet, or downstream of that, surely this would return to the centre tap of the transformer, therefore bypassing the neutral. Under this condition why wouldn't an RCD trip?

 

[123MB]

Well I am not sure if fault current will return as youve said... Since the neutral for the circuit is a different wire from the earth. I initially thought that it would, but the circuit has had an RCD test and the device did not trip.

 

[LionelHutz]

We're going to need to know the exact configuration of this system. In other words, how does this inverter produce an isolated output? How can the scoreboard be 230V L to N connected?

Typically, the center tap ground on the input side would ground the whole inverter install and any fault on the output has the fault path completed by that ground.

 

[123MB]

Hi guys. I can send a drawing in the morning (I am in AU) but to summarise:

1) The output of the inverter is the secondary winding of an isolation transformer
2) This secondary winding is center tapped. You might call it split phase.
3) 240Vac across the secondary winding, no earthed neutral
4) 120Vac from each 'leg' to the earthed center point.

The scoreboard is connected across the secondary winding (230Vac) and extraneous conductive parts of it and other things are connected to the center tapped earth point [highlight #FCE94F][/highlight]

 

[LionelHutz]

OK, I got it now. Just didn't follow through that first explanation.

I believe your standard power system is 230V (or 220V or 240V) with one phase grounded. So, I'm thinking that center tapped transformer should have the center tap left open and one end grounded. I'm not sure how feasible that is though or if doing that would mess with the inverter operation. It sounds like your transformer secondary is wired according to North American single phase 120V/240V standards but AU doesn't use that wiring configuration.

At any rate, if the secondary of the tranformer has a ground then the RCD should be installed on the secondary side of the transformer and it should then trip on any ground fault. I don't believe you need to worry about any RCD protection between the inverter and the transformer.

 

[sibeen]

1) What is the reason for the centre tapped transformer in these arrangements? The output is supposed to be floating, but is it really floating? I am confused…

If the transformer is earthed then it's not a floating system.

Also, I do hope you are protecting both feeds to the control cabinet as both are live, so you'll need a dual ganged circuit breaker.

 

[123MB]

Hi Guys

I have confused myself here. Apparently, the output is NOT a centre tapped transformer.

I concluded that it was a centre tapped transformer because we used a digital multimeter to measure from each line to earth and we got 120VAC.

I have spoken to the inverter manufacturer and apparently the voltage we measured is not actually present - it is a phantom voltage caused by capacitive coupling due to switching noise combined with the use of a high impedance measuring instrument.

I have not confirmed this yet - I am planning on connecting a lamp from each line to earth to confirm that the voltage measured cannot actually energise a load

 

[sibeen]

combined with the use of a high impedance measuring instrument.

Oft times the use of an analogue meter can come in very handy. Wish I still had my old AVO.