Zs measurements in the Cert column

[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.