Bad ZS Readings

[JUD]

Have just seen the amednments and it's now 1533 ohms for final circuits upto 32A and 1667ohms for final circuits over 32A............

Rob...the 1533Ω is for a supply voltage of greater than 230V and less than 400V (230 < Uo ≤ 400).

This is because a circuit not exceeding 32A with a supply voltage within the above range has a max disconnection time of 0.07 seconds or 70mS (Table 41.1 page 46 BRB).

For a 30mA RCD to trip within 70mS it needs a residual current of 150mA (Table 3A page 243 BRB)

230V

 

[rob196]

Are you referring to this:

The alterations seem to have only been made to the > 230V and ≤ 400V column.

The 230V column remains the same.

But the NOT EXCEEDING 32A line has been removed which means it applies to the whole Table, which means the 1667ohms applies to circuits < or > than 32A for 120 V < Uo

230 V.

 

[dave2]

Disconnection time for TT systems can be 0.4 secs providing all equipotential bonding and extraneaous conductive parts are connected and there is an overcurrent device. (These should be connected anyway so 0.2 secs is really nonsense).

Dave

 

[rob196]

Why do we use the voltage at 230? every time i have ever tested the readings have always been over 240 :Salute ...........The other day I was getting readings of 296V on a lighting circuit.....

 

[JUD]

Why do we use the voltage at 230?

Because the voltage range in the UK is 230V +10% -6% (216V to 253V)

 

[Sidewinder]

The other day I was getting readings of 296V on a lighting circuit.....

That's dodgy really!

It should be investigated by DNO as it is outside the ESQCR.

Paul