MAX Zs with 61009

[Carl TDR]

Why do we have maxuim Zs for 61009 RCBO's? I didn't think Zs values really mattered if the circuit was covered by and 30mA RCBO/RCD?

 

[DPG]

Why do we have maxuim Zs for 61009 RCBO's? I didn't think Zs values really mattered if the circuit was covered by and 30mA RCBO/RCD?

It's not right to say the ZS 'doesn't matter', even where there is RCD protection.

 

[Carl TDR]

It's not right to say the ZS 'doesn't matter', even where there is RCD protection.

Yeah fair enough. Can you explain why the Zs is still relevent ?

 

[HappyHippyDad]

Why do we have maxuim Zs for 61009 RCBO's? I didn't think Zs values really mattered if the circuit was covered by and 30mA RCBO/RCD?

I do get a bit confused with when 30mA protection is classed as additional protection and when it is not. However, in the case's where it is classed as additional protection and ADS is the means of fault protection then the RCBO clearly has to meet the max Zs figures otherwise you are not adhering to the regulations.

I think the new attitude towards RCD's being the be all and end all is a bit scary. They are brilliant, but we can't just ignore a Zs reading because of them. A high Zs may also signify some electrical fault and if we get into the habit of placing less emphasis on Zs readings we may be ignoring faults.

 

[DPG]

Nicely summed up HHD. I think you're right - we're heading into the mindset of "I've used an RCD so I won't worry about Zs".
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Yeah fair enough. Can you explain why the Zs is still relevent ?

HHD has covered it nicely in post #5.

 

[Carl TDR]

Okay, that makes sense. The Zs of a circuit can give more information about the state and condition of a circuit.


But what I don’t understand is “Why do we have maximum Zs for 61009 RCBO's?”


As an example:

A TT install has a Ze of 55.0 ohm.

The Maximum Zs for a 63A type C RCBO is 0.27 ohm…

The Zs of the circuit will never meet the maximum Zs of the 61009 according to the regulations?

But we all know that if a maximum touch voltage of 50v / 30mA = 1666 ohm an RCD of 30mA will operate (in theory) with a Ze of 1666 ohm?

 

[cliffed]

The Rcd would provide earth fault protection,& so the 1667 would comply.
The main concern will the short circuit fault conditions be met,these will have to be worked out,with the thermal constraints & the Adiabatic equation,to make sure it then complies.
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Something like this

 

[Carl TDR]

I see... So you use the measured zs value ( or r1 r2 ) to work out the length of the circuit ( of the line an neutral conductor) and then use the Adiabatic equation to calculate if the short circuit fault condition will comply?

Why do we have maximum Zs values in the regs for RCBO's / RCD's ?

Thanks for your help

 

[cliffed]

I see... So you use the measured zs value ( or r1 r2 ) to work out the length of the circuit ( of the line an neutral conductor) and then use the Adiabatic equation to calculate if the short circuit fault condition will comply?

Why do we have maximum Zs values in the regs for RCBO's / RCD's ?

Thanks for your help

Because theses are the values that you should work to,especially on tn systems.
Its second nature these days to forget about these values & stick a Rcd on the high values.But Realistically ,these are the Zs values to be used.
If you use a Rcd then a short circuit calculation shoul also be done to make sure all is ok.