how to prove in a book that the highest x5 of an RCD test is sufficient please?

[Zack123]

Hi, could anyone tell me how to prove in a book that the highest x5 of an RCD test is sufficient please? It’s a question on my am2 testing. Thanks

I’m also struggling on how to check that my measure PFC at DB will trip the breakers in time. This question was a tick box on the exam so if anyone knows how to work this out or a page in a book, that’d be great. Thanks again

 

[telectrix]

the highest of the 2 readings @ x5 must be < 40mSec.

generally the current required to trip a breaker within 0.4 secs. is 5 x In. but checking time/current curves will confirm if pfc is high enough. same for fuses. check the data.

 

[pc1966]

I'm not sure I quite understand your first question. The purpose of the different test currents are to verify that the RCD trip characteristics meet the relevant standards. So 0.5*In should not trip, 1*In should trip in under 300ms (I think), and under 40ms at 5*In. In reality most will trip under 40ms at In anyway.

As for the disconnection times your PFC is of course related to Zs, but fundamentally it is the fault current that causes the OCPD to disconnect. Depending on the type of OCPD and the circuit requirements for disconnection time you can look up the characteristics to find the current needed to disconnect in, say, under 0.4s and if your PFC exceeds that you know your circuit is adequately protected.

E.g. a 32A B-curve MCB needs 5*32A = 160A to guarantee you hit the "instantaneous" magnetic trip (which is usually under 20ms). So if your end of circuit PFC is above 160A then a hard fault will cause it to disconnect fast enough.

Now because it is Zs that is (more or less) fixed, and so PFC = Uo / Zs, when you are testing what you really want to know is "will it always meet the disconnection times?" so what you test for is at minimum Uo (0.95 * 230V) and allowing for cold versus hot conductors (0.8 factor) will my Zs lead to a minimum PFC that meets the disconnection current needed?

Those Zs values are tabulated in the OSG for ease of reference for typical fuses and MCBs.

For MCCB you have to look up the manufacturer's spec and allow for any adjustable settings to be sure, as they are not as standardised as the B/C/D curve MCBs.

 

[radiohead]

I’m also struggling on how to check that my measure PFC at DB will trip the breakers in time. This question was a tick box on the exam so if anyone knows how to work this out or a page in a book, that’d be great. Thanks again

I think possibly you are confusing PFC as you state you are measuring at the DB?.
Prospective short circuit current is measured at the DB or intake to verify that the breaking capacity of protective devices is not exceeded. Measure between live conductors and between live conductors and earth with bonding in place. The highest reading will be the PSCC and is compared to the breaking capacity of protective devices. For example a 60898 MCB normally has a BC of 6ka, so the measured PSCC at the DB would not be expected to exceed this.
As above, PFC at the furthest point of a circuit will determine the operating time of the protective device and is not measured at the DB but at the furthest end of a circuit. This is the Zs reading and determines the fault current.
So a Zs of 1 ohm on a 230v supply will result in a maximum earth fault current of 230a.

 

[Zack123]

the highest of the 2 readings @ x5 must be < 40mSec.

generally the current required to trip a breaker within 0.4 secs. is 5 x In. but checking time/current curves will confirm if pfc is high enough. same for fuses. check the data.

hi thanks for the reply, in the exam it asks for a table to show the trip time is okay, do you know which table in either GN3 OSG or 18th please?

 

[Ian1981]

hi thanks for the reply, in the exam it asks for a table to show the trip time is okay, do you know which table in either GN3 OSG or 18th please?

Appendix 3 in bs7671 for rcd times.
Ito be in the OSG as well but not sure in the page number of the top of my head.

 

[Wilko]

Hi - as per Ian, it’s Table 3A in Appendix 3 of BBB. Put your ruler on 30mA rated residual operating current and read across for times.

 

[Zack123]

I think possibly you are confusing PFC as you state you are measuring at the DB?.
Prospective short circuit current is measured at the DB or intake to verify that the breaking capacity of protective devices is not exceeded. Measure between live conductors and between live conductors and earth with bonding in place. The highest reading will be the PSCC and is compared to the breaking capacity of protective devices. For example a 60898 MCB normally has a BC of 6ka, so the measured PSCC at the DB would not be expected to exceed this.
As above, PFC at the furthest point of a circuit will determine the operating time of the protective device and is not measured at the DB but at the furthest end of a circuit. This is the Zs reading and determines the fault current.
So a Zs of 1 ohm on a 230v supply will result in a maximum earth fault current of 230a.

Thankyou that’s great, I understand it now?
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I'm not sure I quite understand your first question. The purpose of the different test currents are to verify that the RCD trip characteristics meet the relevant standards. So 0.5*In should not trip, 1*In should trip in under 300ms (I think), and under 40ms at 5*In. In reality most will trip under 40ms at In anyway.

As for the disconnection times your PFC is of course related to Zs, but fundamentally it is the fault current that causes the OCPD to disconnect. Depending on the type of OCPD and the circuit requirements for disconnection time you can look up the characteristics to find the current needed to disconnect in, say, under 0.4s and if your PFC exceeds that you know your circuit is adequately protected.

E.g. a 32A B-curve MCB needs 5*32A = 160A to guarantee you hit the "instantaneous" magnetic trip (which is usually under 20ms). So if your end of circuit PFC is above 160A then a hard fault will cause it to disconnect fast enough.

Now because it is Zs that is (more or less) fixed, and so PFC = Uo / Zs, when you are testing what you really want to know is "will it always meet the disconnection times?" so what you test for is at minimum Uo (0.95 * 230V) and allowing for cold versus hot conductors (0.8 factor) will my Zs lead to a minimum PFC that meets the disconnection current needed?

Those Zs values are tabulated in the OSG for ease of reference for typical fuses and MCBs.

For MCCB you have to look up the manufacturer's spec and allow for any adjustable settings to be sure, as they are not as standardised as the B/C/D curve MCBs.

Ahh with you! Okay thank you for your reply. It is greatly appreciated ?

 

[lakie007]

How long did you wait for your results mate? I finished mine yesterday.

 

[Wilko]

Hi, could anyone tell me how to prove in a book that the highest x5 of an RCD test is sufficient please?

Hi - better late than never - I was thinking that you may not have got the exact answer you needed. Which I think is Regulation 643.8 which says in its note the 5x test is the one to use for an additional protection RCD.