generally, with a TT earthing system, you are relying on RCD/s to provide fault protection as the high value of Ze prevents OCPDs operating within specified times. on commercial, a 100mA RCD may be sufficient, but with domestics, 30mA is stipulated.
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E
Engineer54
Unless the 100mA main switch (or MCCB) has a time delay function, the chances are that any 30mA RCD and the 100mA main switch will both trip together on an actual earth fault occurring!!
Just to reiterate what Tel and E54 said.
For a Terra Terra system unless you have Ra values at a suitable level for ADS by circuit breaker, the requirement is that a residual current device is required for earth fault protection.
There is no specification on the mA rating of this RCD, it could be 500mA but obviously 100mA is a good compromise with nuisance tripping and safety.
Of course a 500mA RCd with a 200 ohm Ra would not comply!!!
If there is a requirement on a specific circuit to have additional protection to protect people by using a 30mA RCD then you need discrimination between RCDs and upstream time delayed ones ensure that only the circuit at fault disconnects.
For a Terra Terra system unless you have Ra values at a suitable level for ADS by circuit breaker, the requirement is that a residual current device is required for earth fault protection.
There is no specification on the mA rating of this RCD, it could be 500mA but obviously 100mA is a good compromise with nuisance tripping and safety.
Of course a 500mA RCd with a 200 ohm Ra would not comply!!!
If there is a requirement on a specific circuit to have additional protection to protect people by using a 30mA RCD then you need discrimination between RCDs and upstream time delayed ones ensure that only the circuit at fault disconnects.
Ok thanks completely understand I didn't need to ask this question but I did lol so let's take a 6A lighting circuit on 100mA rcd main switch . If there's a fault it will trip but will it trip before reaches 50v.. I do t even know what I'm asking now. What's the formulae is it uo/Ian i
D
Deleted member 9648
There's no specific requirement in 7671 for an RCD of any sort on a TT system. An RCD is the preferred means of earth fault protection.
The same factors on a TT as on a TN may make a 30ma RCD on a circuit necessary.
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The formula is Ra * Idn <= 50V (this is V= IR for when the voltage is 50V and the current is the rated trip current of the RCD)
and you can meet this by having the maximum Zs for a circuit meeting the table 41.5 values (1667 for 30mA, 500 for 100mA, 167 for 300mA and 100 for 500mA)
So on your 6A lighting circuit if there is a fault to earth then the Zs of that circuit must be less than 500 ohms in order to ensure the 50V touch voltage is not exceeded.
sorry. poorly phrased that. meant that in domestic installations, used by ordinary persons, a 30mA RCD is stipulated for virtually all circuits, and on a TT systems will most likely act as fault protection.
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