Maximum zs generator and rubber box

[davesparks]

The generator when tested produced a fault current of 2.8Ka

The generator is running prime.

The generator has been serviced every 450hours

I have recently started working for this company after 14 years in fixed wiring, but maybe it doesn’t transfer over like fixed wiring?

Is that the actual measured current under fault conditions? Or is that the fault current given by a test meter after a Zs test?

It’s not completely different to fixed wiring, but there are extra considerations with generators.

Generators react to the load applied a lot more than a supply from the DNO does. The DNO supply has several massive alternators in power stations across the country keeping it stable, the generator has just its own engine and alternator.
The engine will slow down when a large load is applied, so they can rarely deliver large instantaneous currents, it takes time to speed up the engine.

You can kind of relate a short circuit to trying to pull away in 5th gear , it don’t like it and will probably stall.

 

[UPS]

That is the value given after doing a test with a meter.

The generator is fitted with a PMG I’m sure this was meant to stop this happening or at least help?

So let’s cut to the chase am I missing something or do the values on the chart still count on a generator fed installation?

Thank you for every ones input.

 

[Risteard]

As above the generator is unlikely to be able to deliver sufficient current into a fault for disconnection either at all or at least within the required time.

 

[UPS]

As above the generator is unlikely to be able to deliver sufficient current into a fault for disconnection either at all or at least within the required time.

This is the value I have from the generator this doesn’t seem dissimilar from what I would expect from a fixed install.

 

[westward10]

PM Generators have been proven to sustain short circuit current during fault conditions which prevents the field collapsing.

 

[Risteard]

View attachment 41625


This is the value I have from the generator this doesn’t seem dissimilar from what I would expect from a fixed install.

That doesn't mean that the generator can actually physically deliver that current into a fault though without the voltage collapsing and the generator stalling. Of course you will have a low impedance to the neutral/Earth link in your own generator - still doesn't mean the generator can provide that fault current.

 

[westward10]

I disagree this is one of the key features of a PMGenerator, sustained output even under fault conditions.

 

[Wilko]

Hi - the meter shows calculation from the measured impedance and V. It doesn't reflect the stored energy in the generator flywheel which would be important if the current was required for 5 seconds, say. IMHO

 

[davesparks]

PM Generators have been proven to sustain short circuit current during fault conditions which prevents the field collapsing.

Not something I’ve ever come across yet, presumably PM = permanent magnet?

Surely the engine is still going to need time to react to the change in load though?

 

[davesparks]

View attachment 41625


This is the value I have from the generator this doesn’t seem dissimilar from what I would expect from a fixed install.

On a generator that value is not much use, that meter calculates the value it doesn’t measure it. It doesn’t take in to account any of the characteristics of the alternator or the engine.