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TT systems and RCD trip times

Sparkymania
Hi all


Just reading through the disconnection times in the regs and noticed something I've never thought of before.


On table 41.1 there is a note under the table that states that the disconnection times of a TT system can be the same as a TN system if disconnection is achieved by an OCPD and bonding is in place. However, I've not come across a TT system with a Ze low enough where only an OCPD can be used. The lowest Ze I've seen was 20 ohms.


Therefore:

On a TT system the disconnection times are 1 second and 0.2 of a second. Reg 411.3.2.2 & 411.3.2.4.

You would normally use an RCD for fault protection due to earth rod resistance causing higher than max Zs on final circuits as well as additional protection. Reg 411.5.3.


However, according to the OSG 11.2 and 11.3 the max allowable time to trip for a 61008 and 61009 can be up to 300ms (or, as stated, less then 300ms so 299.99ms).

If you are using RCBOs on circuits requiring 30mA protection and a selective type on the incoming it is 150 to 500ms on circuits without the RCBOs, so even worse.


This will mean it may not comply with the disconnection times. If you test the RCD and it trips within the 200ms it will comply but the fact that the BS standards allow for a longer time means that technically it does not comply.


(On a side note it seems strange that the older BS4293 RCD specifications do have a max trip time of less that 200ms, although it would still be the same issue with the selective type).


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  • 0
    Sparkymania, have a look at Table 3A on page 363 of 7671. The two requirements for TT systems are that disconnection times must be met and that the product of Ra and I delta n must be less than 50v. If you examine the latter and consider a 100mA RCD is in circuit thus requiring Ra to be nmt 500 ohms. Accepting a relatively low impedance for the DNO earth electrode and the rest of the circuit then the fault current would be circa 230v/500 ohms = 0.46A or 460mA. Refer to Table 3A and you will see that said RCD would be expected to operate nmt 150ms.

    Although this is not the purpose of the second requirement cited above, it does in reality serve to effect disconnection on time.
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  • 0
    Sparkymania, have a look at Table 3A on page 363 of 7671. The two requirements for TT systems are that disconnection times must be met and that the product of Ra and I delta n must be less than 50v. If you examine the latter and consider a 100mA RCD is in circuit thus requiring Ra to be nmt 500 ohms. Accepting a relatively low impedance for the DNO earth electrode and the rest of the circuit then the fault current would be circa 230v/500 ohms = 0.46A or 460mA. Refer to Table 3A and you will see that said RCD would be expected to operate nmt 150ms.

    Although this is not the purpose of the second requirement cited above, it does in reality serve to effect disconnection on time.
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