Maximum zs generator and rubber box

[UPS]

Can anyone point me in the right direction.

I am working for a company who do temp power for welders etc from a generator.

We have been given a load of new boards to pre check befor they go on site. Powerlock connectors in and then D63 on a commando sockets.

I have put the maximum zs of the sockets from the table I usually use. But have just recieved an email from a member of management saying that we need to discuss the max zs values I have used (0.14).

Am I missing something?

 

[Pete999]

Can anyone point me in the right direction.

I am working for a company who do temp power for welders etc from a generator.

We have been given a load of new boards to pre check befor they go on site. Powerlock connectors in and then D63 on a commando sockets.

I have put the maximum zs of the sockets from the table I usually use. But have just recieved an email from a member of management saying that we need to discuss the max zs values I have used (0.14).

Am I missing something?

Well yes you are, you have only noted the Maz Zs for the particular OCPD you are using, not the actual Zs of the instalation, Zs = Ze+ (R1+R2) So a Zs measurement needs to be taken at the point of utilisation, ergo the socket you are supplying.

 

[Wilko]

Not an expert here but to add to @Pete999 thoughts - would the gen set be able to provide the 1260A required to trip a D63 instantly or even 630A for 5 sec? So, perhaps it's earth fault protection is via RCD?

 

[Pete999]

Not an expert here but to add to @Pete999 thoughts - would the gen set be able to provide the 1260A required to trip a D63 instantly or even 630A for 5 sec? So, perhaps it's earth fault protection is via RCD?

Yeah could be Wilko

 

[Pete999]

Can anyone point me in the right direction.

I am working for a company who do temp power for welders etc from a generator.

We have been given a load of new boards to pre check befor they go on site. Powerlock connectors in and then D63 on a commando sockets.

I have put the maximum zs of the sockets from the table I usually use. But have just recieved an email from a member of management saying that we need to discuss the max zs values I have used (0.14).

Am I missing something?

Is the circuit orotected by the D63 a distribution circuit, if so it could be argued that the Zs talked about, coul be the ZS@the DB (Ze of the distribution circuit) or am I overthinking the issue, have been acused of that before.

 

[davesparks]

Are these standard rental generators?
Normally you’d be using the variable earth leakage protection on the generator set and any other earth leakage protection in your rubber boxes for your earth fault protection. The ocpds will only be providing over current protection.

Generators rarely have the ability to deliver the fault currents needed to use the mcb for fault protection. Quite often the generator will stall or shutdown on engine under-speed before any of the protective devices operate.

 

[UPS]

The generator is a 800kva and the earth leakage is left off.

The gen can produce the required current to trip the breaker on instantaneous.

The circuit is not a disto circuit it is being used to feed welding equipment.

 

[davesparks]

Well yes you are, you have only noted the Maz Zs for the particular OCPD you are using, not the actual Zs of the instalation, Zs = Ze+ (R1+R2) So a Zs measurement needs to be taken at the point of utilisation, ergo the socket you are supplying.

Generators don’t strictly have a Ze, I usually see it expressed as Zgen if anything.

 

[davesparks]

The generator is a 800kva

800kVA prime or standby?
What is the maximum fault current it can deliver and in what time?
How recently was the engine serviced and is it in full working order?

There’s more to consider with a generator supply than there is with a DNO supply.

 

[UPS]

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?

 

[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.

 

[UPS]

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.

So how do I know that I am going to achieve disconnection times required?

 

[davesparks]

So how do I know that I am going to achieve disconnection times required?

You’ll need to characteristics of the generator to establish the available fault current by calculation or from the data sheet for the generator.
For all the generator setups I am involved with I use variable earth leakage relays to achieve fault protection.

 

[LeeH]

I’m going to embarrass myself here and ask what a rubber box is.

?

Great posts from Davesparks btw.

 

[UPS]

I’m going to embarrass myself here and ask what a rubber box is.

?

Great posts from Davesparks btw.

Just like a waterproof box with sockets mounted on it.

 

[davesparks]

I’m going to embarrass myself here and ask what a rubber box is.

?

Great posts from Davesparks btw.

Rubber box is a company who manufacture distribution units (usually referred to as distro) for temporary installations, usually made of tough rubber.
The term rubber box has come to mean any rubber distro, much the same as vacuum cleaners being referred to as hoovers.

 

[shaun1]

You’ll need to characteristics of the generator to establish the available fault current by calculation or from the data sheet for the generator.
For all the generator setups I am involved with I use variable earth leakage relays to achieve fault protection.

Out of interest, how do you protect against Line to neutral faults?