Discuss Will a reading higher than given in 7671 for Zs values be okay if protected by RCD in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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J

joeyscurlock

I might be COMPLETELY wrong.... but I have recently finished my level 3 in college and I'm sure I was told that if a Zs value is higher than the max reading in BS7671 it's okay if it's protected by a 30mA RCD, so it passes. I sort of remember writing 'satisfactory' on the test form next to Zs reading that are protected by RCD then checking results against circuits not protected by an RCD. The lads I work with are determined that's not true because RCDs protect against earth leakage and MCBs protect against overload and Zs readings are for overload.. Which i also thought was wrong because I thought Zs is earth fault loop path which ties in with RCD protection. I'm not certain because of the experience they have compared to me, but i'm sure some of what i've said is true.

If anyone could answer this for me that would be great.
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

for a circuit protected by a 30mA RCD the max. Zs is 1667, but a circuit should be designed so as the Zs is below the figures stated for the MCB./s
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

Surely an overload on the circuit would incur between line and neutral and the Breaker will take however long to operate, depending on the amount of overload. Zs is for earth fault and the impedance should be low enough for disconnection times to be met for given circuit.
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

It all boils down to why the Zs is higher. Is it due to the design of the circuit or because theres a fault on the circuit.
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

Surely an overload on the circuit would incur between line and neutral and the Breaker will take however long to operate, depending on the amount of overload. Zs is for earth fault and the impedance should be low enough for disconnection times to be met for given circuit.

bear in mind that Zs values relate to L-E fault protection, not L-N.
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

for a circuit protected by a 30mA RCD the max. Zs is 1667, but a circuit should be designed so as the Zs is below the figures stated for the MCB./s

Only applies to a TN earthing system.
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

Yeah sure , so between Line and Neutral the MCB would operate under short circuit. Btw am still only training myself just about finished the first year of 2357 so still got lots to learn.
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

Is that not the value of a 30mA RCD operating as the 1667 value is the touch voltage value !
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

7667 is the value at which the RCD will trip ( 230V/0.03A). the 1667 is 50V/0.03A so as to trip before touch voltage exceeds 50V.
 
Re: Will a reading higher than given in 7671 for Zs values be okay if protected by RC

7667 is the value at which the RCD will trip ( 230V/0.03A). the 1667 is 50V/0.03A so as to trip before touch voltage exceeds 50V.

I know that, my point still stands though. 411.4.9 Tel, on TN systems Zs x Ia must be equal to or less than Uo (not 50V).

7667 ohms is the maximum allowable Zs on a TN system where a 30mA RCD has been used to provide earth fault protection :)
 

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