Testing of DC/AC inverter fed Distribution board

[Mwgs]

Hi guys,
We have upgraded a DC supplied crane with a 460V DC to 230V AC inverter to supply a D/Board for lighting and 110V transformer for tools etc. The question I have is as the Inverter is self regulating its max ouput to 38Amps it will never produce fault current to trip devices as per BS7671, also how do you measure Ze if its going into an electronic inverter as the gubbings inside will surely affect the readings?

Apologies if this sounds silly:thinking2:

Thanks

 

[westmac]

There will be no Ze that you can measure no, if you contact the manufacturer they should be able to provide you with some sort of document which states the devices disconnection times in the event of both a short circuit or earth fault. My NICEIC inspector was happy to accept this as the install beyond the inverter falls under a BS number other than BS7671.

 

[Mwgs]

Thanks westmac,

The Inverter actually limits itself when it goes overcurrent by cycling the output, although it does give an alarm signal via relay which I have used to interupt the outgoing MCB with an Undervolt trip. The issue being the customer is adamant that this needs to comply with BS7671 disconnection times which clearly we cannot provide. What would the BS number be for the install other than 7671? maybe I could work through this and suggest an alternative test for him.

 

[IQ Electrical]

As westmac said, the UPS itself will clear any faults by limiting it's own short circuit current and eventually shutting down-often at around 100mS to protect the inverter and any damage downstream caused by the fault.
The manufacturer will confirm this and you will no doubt meet relevant BS7671 disconnection times.

Any 'Zs' values you measure are only referencing the impedance of the output stage of the UPS and are therefore meaningless as far as disconnection times are concerned.

 

[westmac]

Thanks westmac,

The Inverter actually limits itself when it goes overcurrent by cycling the output, although it does give an alarm signal via relay which I have used to interupt the outgoing MCB with an Undervolt trip. The issue being the customer is adamant that this needs to comply with BS7671 disconnection times which clearly we cannot provide. What would the BS number be for the install other than 7671? maybe I could work through this and suggest an alternative test for him.

The only way to "test" the various operation times of the built in safety systems would be with some very expensive electronics. Don't forget that BS7671 requires you to make the installation safe (put briefly), it never actually stipulates how this should be achieved. I would strongly suggest that you contact the manufacturer and explain your situation, ours tried to fob me off at first but eventually they'll help you out.

If it is even a half decent piece of kit, the operation times will make a mockery of ADS systems we install day in day out in terms of real disconnection times. You'll be talking thousandths of a second rather than tenths.

If you're customer still will not accept the facts then get some stationary out and draw a typical earth fault loop path, once they understand this you can explain that your inverter is essentially a micro power station generating an artificial voltage source and no electrician will have the kit to test this.

The only tests you can carry out are visual checks that the cable is capable of carrying the current as per the "overload" setting of the inverter and insulation resistance tests.