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RCD disconnection times on a TT system

Hi Guys, first post after a long time reading the forums.

I have a client questioning the disconnection times provided by a 30mA type A RCBO on a TT system, tripping times provided are 221ms at 30mA and below 40ms at 150mA.

It protects a 16A socket in an outdoor environment, so must comply with the disconnection time provided in Table 41.1 of BS7671 of 0.2 seconds

Looking at table 3A on page 363, that states that a 30mA RCD must trip within 300ms at 1x, which is obviously outside the limits above, and 40ms at 5x.

Can we state that because the 5x test trips at less than 40ms it complies with fault protection, or does the 30mA disconnection time also have to comply?

Thanks in advance for any guidance you can offer.

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0 Volt Hunter over 5 years ago

Thanks for all the replies! I had to go out on site Friday so didn't have a chance to read through until this morning.

To answer some of the questions, it wasn't ourselves that tested it so have to reply on the customers response:

The switchgear is all Eaton type A RCBOs

The RCBO only supplies an outdoor 16A socket, so no loads were connected at the time of test.

There are numerous other RCBOs of the same type, all giving timings of 160 to 199ms.

All Zs readings are "well below maximum permitted values"

They were tested with multiple test instruments, getting the same readings, so were cycled many times.

disconnection will occur within the required time for a L-PE fault of negligible impedance.

I had missed those last two words, so as the 5x tests are all under 40ms I don't think we have a problem.

Many thanks again
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0 Volt Hunter over 5 years ago

Thanks for all the replies! I had to go out on site Friday so didn't have a chance to read through until this morning.

To answer some of the questions, it wasn't ourselves that tested it so have to reply on the customers response:

The switchgear is all Eaton type A RCBOs

The RCBO only supplies an outdoor 16A socket, so no loads were connected at the time of test.

There are numerous other RCBOs of the same type, all giving timings of 160 to 199ms.

All Zs readings are "well below maximum permitted values"

They were tested with multiple test instruments, getting the same readings, so were cycled many times.

disconnection will occur within the required time for a L-PE fault of negligible impedance.

I had missed those last two words, so as the 5x tests are all under 40ms I don't think we have a problem.

Many thanks again
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RCD disconnection times on a TT system

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