Testing portable welfare units with built in generator

[isk57]

Hi
I’ve recently tested a unit and rcd times are fine and R1R2 but Ze and Zs are High values and are not within bs7671 for a tns supply , I don’t really know what ze I should be getting to start with and would it be better treating it as a TT ,I will admit I’ve never done one before and only done houses

 

[Stevesja]

Hi
I’ve recently tested a unit and rcd times are fine and R1R2 but Ze and Zs are High values and are not within bs7671 for a tns supply , I don’t really know what ze I should be getting to start with and would it be better treating it as a TT ,I will admit I’ve never done one before and only done houses

I do a lot of mobile medical units that have either built in or stand alone generators.
Dont forgot a generator is not going to give you the best Zs as it has a high impedance because of how its built and works.
Dealing with generators in the 7909 world small sets can be set up diffrent ways, INS, TNS but rarely TT and unless your deploying a rod for a true TT rather than for a TNS dont think of treating it as TT.
Most built in small sets wont be able to deliver enough fault current for the CBs it will just stall instead. There is a big reliance on the RCD. Have a look in 7671 at mobile and transportable units also 7909 if you have it or one of the guides to 7909 theres some really useful generator diagrams aimed towards OB units mainly but a lot of the principles are the same.
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I do a lot of mobile medical units that have either built in or stand alone generators.
Dont forgot a generator is not going to give you the best Zs as it has a high impedance because of how its built and works.
Dealing with generators in the 7909 world small sets can be set up diffrent ways, INS, TNS but rarely TT and unless your deploying a rod for a true TT rather than for a TNS dont think of treating it as TT.
Most built in small sets wont be able to deliver enough fault current for the CBs it will just stall instead. There is a big reliance on the RCD. Have a look in 7671 at mobile and transportable units also 7909 if you have it or one of the guides to 7909 theres some really useful generator diagrams aimed towards OB units mainly but a lot of the principles are the same.

 

[davesparks]

Hi
I’ve recently tested a unit and rcd times are fine and R1R2 but Ze and Zs are High values and are not within bs7671 for a tns supply , I don’t really know what ze I should be getting to start with and would it be better treating it as a TT ,I will admit I’ve never done one before and only done houses

Ze is the EFLI of the supply external to the installation. There is no such thing for a self contained unit so you cannot have measured Ze.

Yes Zs can be high if the generator is small or very close to the point at which you are measuring. Consult the specs of the generator to find out what fault current it can deliver etc.

What exactly do you mean by 'treat it as a TT'? If the generator is built in to the unit how and why would you try to make it a TT system??