Discuss Zs measurements in the Cert column in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net
What would you put.and see if can get correct.what Max Zs would you put in the column for Max measurement for the MCB
Are you testing TT or PME.I would put 1666 for the Rcd measurement & the normal Zs measurements for the MCB’s
PME CheersAre you testing TT or PME.
in your tick box for Max measurement tested.
Well that’s the dilemma & some disagreement here… To be fair I’ve seen both measurements recorded on Certs… personally I prefer the Zs for the MCB, with the 1666 only for the MCB1667 ohms for any circuit protected by a 30mA RCD
I personally don't see any value in using the MCB max permitted Zs values when there is an RCD protecting the circuit. Following 411.4.204, table 41.5, and 643.7.1 (a) and (b) means that I don't have to provide a Zs value, either measured or calculated, to prove ADS, assuming I have confirmed continuity of CPC. It frees me up to focus on other parts of the testing.Well that’s the dilemma & some disagreement here… To be fair I’ve seen both measurements recorded on Certs… personally I prefer the Zs for the MCB, with the 1666 only for the MCB
Yea that’s my thinking, scares me looking @ those 1666 measurementsI would argue that the RCD component of the circuit is there for additional protection - therefore you'd always use the appropriate Zs to achieve a disconnection time under ADS except if the default stated values by the manufacturer are those for RCD anyway.
In theory, a max impedance of 1666 on a 230v nominal supply does give you a current of slightly less than 5x but vastly exceeds 1x so it shouldn't matter..... but ..... we press the test button on RCD's for a reason whereas ADS just dependably sits there.Yea that’s my thinking, scares me looking @ those 1666 measurements
Agree so why would anyone wanna put 1666 in the column regarding TN SystemsIn theory, a max impedance of 1666 on a 230v nominal supply does give you a current of slightly less than 5x but vastly exceeds 1x so it shouldn't matter..... but ..... we press the test button on RCD's for a reason whereas ADS just dependably sits there.
But frankly if you're even anywhere close to working with numbers that high then you've got bigger issues anyway, not least of which is Vd.
I think the technical argument there is that the RCD would be additional protection not basic protection under ADS??If ADS is satisfied by the mcb then use the mcb If ADS is satisfied by the RCD then use that.
415.1.2 The use of RCDs is not recognized as a sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414.415.1.2 just means if you have an EN 61008 device then you must have an overcurrent device incorporated into it, l believe.
Why? Because that is the maximum Zs for the circuit. The RCD doesn't know or care whether the earthing arrangement is TN or TT, or if you prefer the values for MCB, it's going to trip anyway, and that is its max Zs.Agree so why would anyone wanna put 1666 in the column regarding TN Systems
Especially when the actual measured Zs is going to be a low Ohm reading
I find this odd but it is the way of things now. If you have your R1 + R2 you may as well put in the Zs anyway by way of calculation.Also: Because where a circuit is protected by an RCD, there is no need to provide a Zs reading, either measured or calculated. Once you have proved that the RCD works, and that the CPC is continuous by either R2 or R1+R2, then you have met disconnection times.
Why bother? It's not useful data, as you can only check it against 1667ohms.I find this odd but it is the way of things now. If you have your R1 + R2 you may as well put in the Zs anyway by way of calculation.
While that is technically true it is a path that leads to ECIR by socket tester results - wet string below 1.6k ohm? Pass!Why? Because that is the maximum Zs for the circuit. The RCD doesn't know or care whether the earthing arrangement is TN or TT, or if you prefer the values for MCB, it's going to trip anyway, and that is its max Zs.
Also: Because where a circuit is protected by an RCD, there is no need to provide a Zs reading, either measured or calculated. Once you have proved that the RCD works, and that the CPC is continuous by either R2 or R1+R2, then you have met disconnection times.
While I hear what you're saying, I think that focussing on Zs is the wrong thing to focus on, when time can be better spent on other parts of I and T.While that is technically true it is a path that leads to ECIR by socket tester results - wet string below 1.6k ohm? Pass!
My own view is you should be looking at an installation to see if it is in good safe condition, and if you are not meeting Zs on a TN system it starts to ring alarm bells. It could just be a long cable (or high current distribution circuit and high-ish supply Ze) so it is healthy but reliant on the RCD for ADS within the allocated time, or it could be an indication that something is badly corroded and might not survive more than one fault or a bit of vibration before it goes open and the person becomes the path for ADS current to trip things.
Also I get a touch nervous at the use of a single RCD for your ADS. They are rarely tested outside of ECIRs (even if they should be done twice a year or more), and the electronics is many times more complex than the thermal-magnetic trip of the MCB side of things. Yes, it is perfectly within the regs, but having seen the odd failed RCD in the past I would really prefer to have two RCDs (delay incomer & additional 30mA ones) so there is no single point of failure in ADS if something like TT earthing made it the only practical means of achieving ADS.
Yes, 1667 for the max permitted. If I have measured the Zs, then it goes in the max measured, but generally this isn't necessary for RCD protected circuits, in which case I leave it blank.So presumably you put 1667 for all RCBO then @Pretty Mouth, as the Max but put the measured Zs in the other column?
There are no requirements in ADS to disconnect for a L-N fault.But splutter splutter, why isn't a Zs generally necessary? Not saying you're wrong but curious to know your reasoning as to why you are not really bothered about ensuring the MCB L-N fault is within ADS requirements?
Reply to Zs measurements in the Cert column in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net
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