Zs measurements in the Cert column

[cliffed]

Still having a discussion @ work about this…TNCS system DB/CU is a split 30ma RCD configuration,5/5
MCB’S are 60898 what Max Zs would you put in the column for Max measurement for the MCB

 

[buzzlightyear]

what Max Zs would you put in the column for Max measurement for the MCB

What would you put.and see if can get correct.

 

[cliffed]

I would put 1666 for the Rcd measurement & the normal Zs measurements for the MCB’s

 

[buzzlightyear]

I would put 1666 for the Rcd measurement & the normal Zs measurements for the MCB’s

Are you testing TT or PME.
in your tick box for Max measurement tested.

 

[cliffed]

Are you testing TT or PME.
in your tick box for Max measurement tested.

PME Cheers

 

[cliffed]

It’s actually in the column Max permitted Zs for installed device ie: MCB 60898

 

[buzzlightyear]

It’s actually in the column Max permitted Zs for installed device ie: MCB 60898
What the regs say what mcb you have installed.

 

[Pretty Mouth]

1667 ohms for any circuit protected by a 30mA RCD

 

[pc1966]

On a TN system I would normally base it on the MCB Zs value.

Yes, you can disconnect according to regs on 1667 ohms with 30mA RCD, but on TN-C-S it ought to be pretty low and unless it is an unusual situation, anything above the usual OSG sort of tabulated values suggests a fault in the circuit implementation which needs fixing, not a planned use of the RCD.

High current sub-main, or TT earthing, are a very different kettle of fish.

 

[cliffed]

1667 ohms for any circuit protected by a 30mA RCD

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

 

[Pretty Mouth]

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

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.

While I agree that a Zs test might show a problem with a loose connection in the circuit, I find the results to be inaccurate when compared to a continuity test (particularly no trip tests and/or TT earthing). IMO, better to take an R1+R2 reading, which should give a more accurate picture of the circuit's health.

EDIT: it also cuts down on the amount of live testing

 

[Vortigern]

I would put in the max for the MCB. It is often the case that the RCD does not trip under testing due to no-one ever testing the RCD since it was installed. If the RCD is inoperable I would like to know the MCB will achieve ADS as per regs despite faulty RCD. I have quite often found RCD not operating, a quick few switches on and off "cures" this, but it was faulty until then.

 

[Rockingit]

I 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.

 

[cliffed]

I 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.

Yea that’s my thinking, scares me looking @ those 1666 measurements

 

[Rockingit]

Yea that’s my thinking, scares me looking @ those 1666 measurements

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.

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.

 

[cliffed]

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.

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.

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

 

[westward10]

If ADS is satisfied by the mcb then use the mcb if ADS is satisfied by the RCD then use that.

 

[Rockingit]

If ADS is satisfied by the mcb then use the mcb If ADS is satisfied by the RCD then use that.

I think the technical argument there is that the RCD would be additional protection not basic protection under ADS??

411.x states in several places that where an RCD is used for additional protection it needs to conform to the same ADS times as for Zs measured circuts (paraphrasing!). HOWEVER, 415.1.2. tells us that "The use of RCDs is not recognised 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".

However again, 411.8.3 eventually tells us "where fault protection is provided by an RCD, the product of the rated residual operating current (I∆n) in amperes and the Earth Fault Loop Impedence in ohms shall not exceed 50V". Which as we all know is where we get our magical 1667 * 0.03 = 50v.

On that basis, I think it's probably (as I answer my own theorem...) more technically correct to use the 1667Ω figure in the max Zs column. Still doesn't mean I like it!!! If one of us could be arsed to pick up a GN3 we'd probably find there's a note in there somewhere that clarifies this!!

 

[westward10]

415.1.2 just means if you have an EN 61008 device then you must have an overcurrent device incorporated into it, l believe.
Where an mcb cannot satisfy ADS it is perfectly acceptable to use an RCD and in some circumstances it will provide additional and fault protection.

 

[Pretty Mouth]

415.1.2 just means if you have an EN 61008 device then you must have an overcurrent device incorporated into it, l believe.

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.

I think it means that we cannot rely on an RCD alone as a complete protective measure against electric shock.

There are:
411 - Protective measure: ADS
412 - Protective measure: Double/reinforced insulation
413 - Protective measure: Electrical separation
414 - Protective measure: ELV/SELV/PELV,

but there is no section called Protective measure: RCD. This, I believe, is what 415.1.2 is reminding us.

 

[Pretty Mouth]

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

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.

 

[westward10]

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.

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.

 

[Pretty Mouth]

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.

Why bother? It's not useful data, as you can only check it against 1667ohms.

 

[westward10]

I like filling boxes

 

[pc1966]

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 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.

 

[Pretty Mouth]

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.

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.

A measured Zs value will prove only one thing - that the disconnection times are met. True, it might give a loose indication that something is wrong, but then again it might not. For example, a measured Zs of 5 ohms on a B6 lighting circuit is well within Zs limits, but there is something amiss if you're getting this in the average domestic property. Or perhaps an immersion heater circuit gives a very nice low Zs, but is using the pipework as an earth.

It doesn't prove that the CPC is continuous, or that the L or N don't have loose connections in them, or give a circuit length, or even that the polarity is correct. These things are much better tested for by continuity.

Yes, sometimes RCDs do fail. However, even if it should fail, it is very likely that the MCB will disconnect anyway, seeing as we calculate max Zs based on the absolute upper limit of the device and a voltage that is far lower than normally encountered.

 

[SparkySy]

Some interesting comments!

Also a high Zs could be caused by RCD uplift, especially with older RCD's.
I put the Max Zs in as that of the 60898 max reading for RcBO's, as above they can fail and then you are reliant on the MCB side of the device for protection.
Sy

 

[Vortigern]

So presumably you put 1667 for all RCBO then @Pretty Mouth, as the Max but put the measured Zs in the other column?

 

[Pretty Mouth]

So presumably you put 1667 for all RCBO then @Pretty Mouth, as the Max but put the measured Zs in the other column?

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.

 

[Vortigern]

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?

 

[Pretty Mouth]

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?

There are no requirements in ADS to disconnect for a L-N fault.

ADS is section 411, in chapter 41: protection against electric shock. L to N faults don't generally cause electric shock, as they cause no rise in voltage on exposed conductive parts, so the disconnection times in 411 do not apply. There are other reasons to disconnect (adiabatic), but these faults aren't required to meet the times tabled in 41.1

411.3.2.1
Except as provided by Regulation 411.3.2.5, a protective device shall automatically interrupt the supply to the line conductor of a circuit or equipment in the event of a fault of negligible impedance between the line conductor and an exposed-conductive-part or a protective conductor in the circuit or equipment within the disconnection time required by Regulation 411.3.2.2, 411.3.2.3 or 411.3.2.4.

(* L to N faults don't generally cause electric shock, as they cause no rise in voltage on exposed conductive parts: actually, I believe that this isn't the full story. I believe that a L-N fault would see a rise in voltage on the earthing system for a TN-C-S earthing arrangement, the E and N being connected at the cutout.

However, a fault so low that it failed to disconnect the protective device quickly would be a long way downstream of the cutout. I believe that much of the voltage would have dropped between the fault and the cutout, so the rise on the earthing system would be low.)

 

[Vortigern]

Thanks for that, good line of thought. My take is that safety is about protection of life/people/livestock hence the earth thing and protection of property. While I certainly agree that usually L-N faults don't normally present danger in terms of electrocution/shock there is a risk of fire/damage to property and hence life if someones in the house at the time of a fire. Therefore I tend to keep an eye on PSSC for the above reasons so I can feel assured that in the event of a fault safety to property etc. is assured. I note your point about N-E connection at the cut-out and often ponder on that and as we know a broken neutral exterior to the site means a live earth and so on. I try not to over-think it all though. Just being as careful as I know how. In any event when doing a Zs L-E the L-N is automatic and presented on the meter so it is not a big deal to note it.