Tera Tera

[i=p/u]

In a TT system in commercial premises with 100mA main switch , does all circuits need to be 30mA rcd protected... Can't think of the sums for this 1. But I will say yes.

 

[i=p/u]

Don't won't to get books out as off rest of week

 

[telectrix]

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.

 

[markc123]

What the resident scouse said

 

[7029 dave]

Ditto post 3

 

[Guest55]

in a commercial install , i'd only fit 30ma rcds on gen. purpose skts and circuits within a location containing bath/shower with a time delay unit upfront.
try and avoid rcd's for IT , lighting and building services circuits if possible.

 

[Engineer54]

In a TT system in commercial premises with 100mA main switch , does all circuits need to be 30mA rcd protected... Can't think of the sums for this 1. But I will say yes.

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!!

 

[Richard Burns]

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.

 

[i=p/u]

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

 

[i=p/u]

50v=ra something something something...

 

[Deleted member 9648]

In a TT system in commercial premises with 100mA main switch , does all circuits need to be 30mA rcd protected... Can't think of the sums for this 1. But I will say yes.

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.

 

[Deleted member 9648]

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.

Where?

 

[Richard Burns]

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

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.

 

[telectrix]

Where?

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.

 

[i=p/u]

Well banged in 7 earth rods today (4+3) and got ze of 14 ohms .. Just thought let you know, if you give a dam or not.. But I was impressed.