Designing a supply with a 0.8 TNS Ze.

[01laverickd]

Hi, first question on here in a very long time, I am hoping for some nice replies ?.

It's been playing on my mind all week and can't seem to find the answer anywhere I'm hoping this is the right place.

I want to run an armoured cable from a TNS supply to a consumer unit, the consumer unit is over 3metres so would be installed with an 60A switched fuse, the main cutout is 100A. (tails from DP isolator, from isolator into switched fuse unit, then swa from switch fuse to consumer unit)

It is a TNS supply and unluckily the Ze reading is 0.8ohms. This reading already exceeds the allowed for the 1361-2 fuse of 0.54 for 5 second disconnection. (80% OSG values).

Where do I stand here? Would I have to provide fault protection with an RCD? for example a 100ma stype and note it on the test sheet? So, swap the 100amp main switch in the switch fuse with a 100ma 100amp rcd?

Going forward, when designing should I always design with the higher 0.8 ze figure for all my tns work, as in theory although extremely unlikely it could rise to that and be OK. Which would have adverse affects on the installation? And likewise 0.35 for a TNCS?

I've always took a Ze measurement at the origin before designing and used that but I've been probably over thinking and can't figure out what to do.

Be nice please haha.

Cheers

 

[Strima]

I think an RCD is your only option, looking at the numbers the nearest fuse size that's within is 32 amp BS88-3.

 

[01laverickd]

I think an RCD is your only option, looking at the numbers the nearest fuse size that's within is 32 amp BS88-3.

I've read somewhere about an omition to the dnos fuse, but once I've installed the switch fuse that I have to it then becomes my responsibility and the omition is no longer.

In essence I'd have to do what I would do on a TT I think? With the 100ma providing fault protection and the fuse for overload.

 

[Strima]

Even though the Zs for the circuit would be higher than recommended I will still retain the TNS earthing arrangement, why get use a TT when there's still a good earth path which is stable?

 

[Wilko]

Hi - I’d instal the new tails so that a 30mA RCD is not required, that leaves fault protection - 100mA S will do that. Fault protection for the final circuits will need to be by additional 30mA rcd or rcbo. I’d still keep using the TNS, the best TT I’ve done was 8 Ohms .

 

[westward10]

I don't believe he intends to make it TT. He is suggesting circuit protection as if it were TT due to the high Ze.

 

[01laverickd]

Even though the Zs for the circuit would be higher than recommended I will still retain the TNS earthing arrangement, why get use a TT when there's still a good earth path which is stable?

Sorry for any confusion, I'm not going to convert into a TT. I was basically saying that I am going to have to do as I would for a supply cable to a consumer unit on a TT system. I was referring to installing a 100ma s type rcd for fault protection alongside the switch fuse. (replace the 100 main switch in the switch fuse with a 100mA s type rcd) for fault protection.



My next question ; do I need to design circuits from now on with the maximum Ze allowed, ie 0.8 TNS incase of the Ze increasing over the years or will I be OK just staying with designing the circuit to measure Ze at that time? If I designed all TNS circuits with 0.8 I'll find myself having to use RCDs for fault protection aswell as additional protection which of course I'd rather use the Mcb for fault protection where possible.

Thanks for the replies so far.

 

[pc1966]

Your 0.8 ohm Zs is not going to impact on too many circuits, though your SWA feed, etc, will add to it, so the concern is really for circuits above, say, 32A where achieving 0.4s disconnect might be difficult on the MCB side of things even with the usual B-cure.

Unless the job is really being done down to a price, then I would be going for RCBOs anyway for everything that is not safety-critical (so unless they have medical stuff, etc) in which case you might meet disconnection on the MCB even for a 45A MCB shower feed, but you should still be compliant and achieve selectivity with the 100mA S-type incomer on the RCD side of things.

I would always try to achieve ADS on Zs as I don't have that high a faith in RCDs over the long-term as practically nobody tests them outside of EICRs when folk move house, etc. But there are cases like this where you don't have the option.

Longer term I suspect if anything Zs would come down as most likely the DNO will change to TN-C-S at some point.

 

[01laverickd]

Your 0.8 ohm Zs is not going to impact on too many circuits, though your SWA feed, etc, will add to it, so the concern is really for circuits above, say, 32A where achieving 0.4s disconnect might be difficult on the MCB side of things even with the usual B-cure.

Unless the job is really being done down to a price, then I would be going for RCBOs anyway for everything that is not safety-critical (so unless they have medical stuff, etc) in which case you might meet disconnection on the MCB even for a 45A MCB shower feed, but you should still be compliant and achieve selectivity with the 100mA S-type incomer on the RCD side of things.

I would always try to achieve ADS on Zs as I don't have that high a faith in RCDs over the long-term as practically nobody tests them outside of EICRs when folk move house, etc. But there are cases like this where you don't have the option.

Longer term I suspect if anything Zs would come down as most likely the DNO will change to TN-C-S at some point.

Thanks for your detailed reply PC.

Yes I agree it won't make much difference but if I was working out my designs to a 0.8 ze for tns I may aswell just put a 100ma s type rcd on every TNS quote?

I would normally always use Rcbos to be honest. I'm lost as to how i might be able to achieve disconnection through the mcb though if I used Rcbos? Surely the max zs is the same? (might be having a blonde moment ?)


Yes that is always what I try if i can to achieve ADS through the Zs.


Thanks again.

 

[pc1966]

Here you are looking at the up-front S-type RCD due to the sub-main protection issue. If you only had the problem of a high Zs on a normal CU install then you are looking at either keeping the MCB trip points down, or depending on the RCD action of RCBOs.

Zs of 0.8 ohms is the upper limit usually assumed for TN-S, it can be more but i think something like 90% will be less. If they don't have any high current circuits you are OK even on the OCPD design, and what used to be done was to use a smaller fuse on any rings if you had a high feed Zs (e.g. 20A or 25A).

In theory your typical 30mA RCBOs are OK for Zs below 1667 ohms, but in reality you would not trust that as earth rods should be less than 200 ohms to be considered stable enough given seasonal changes in soil moisture and freeze risk.

So while the RCD side is your means of escaping from high Zs, it is still not really a good approach if you can meet disconnection on the MCB side as it is much simpler and more reliable than the electronics needed for the RCD function.

A quick look at the OSG table B6 shows that type B MCBs are OK for 0.8 ohms Zs up to 40A, then you start to have issues, so high power showers or cookers needing 45A or 50A supplies are your risk here depending on the RCD. Now you might decide to return to the 100mA incomer here, as even if the RCBO has failed you would have the back-up of the main incomer RCD to trip in the worst-case so no single point of failure.

But in practice the OSG is the worst-case: for lowest supply volts, highest magnetic trip tolerance, and 0.8 factor for hot cable resistance. If you take the average values you have 4 * 45A = 180A instant trip for a 45A B-curve MCB hence with a common 240V supply your Zs limit is actually 1.3 ohms if the fault happens on switch-on. Basically the risk is not quite so bad as initially appears, though for compliance with the regs you should be doing it worst-case or relying on RCD action (which is permitted, but something I kind of like to avoid as above).

 

[marcuswareham]

Here you are looking at the up-front S-type RCD due to the sub-main protection issue. If you only had the problem of a high Zs on a normal CU install then you are looking at either keeping the MCB trip points down, or depending on the RCD action of RCBOs.

Zs of 0.8 ohms is the upper limit usually assumed for TN-S, it can be more but i think something like 90% will be less. If they don't have any high current circuits you are OK even on the OCPD design, and what used to be done was to use a smaller fuse on any rings if you had a high feed Zs (e.g. 20A or 25A).

In theory your typical 30mA RCBOs are OK for Zs below 1667 ohms, but in reality you would not trust that as earth rods should be less than 200 ohms to be considered stable enough given seasonal changes in soil moisture and freeze risk.

So while the RCD side is your means of escaping from high Zs, it is still not really a good approach if you can meet disconnection on the MCB side as it is much simpler and more reliable than the electronics needed for the RCD function.

A quick look at the OSG table B6 shows that type B MCBs are OK for 0.8 ohms Zs up to 40A, then you start to have issues, so high power showers or cookers needing 45A or 50A supplies are your risk here depending on the RCD. Now you might decide to return to the 100mA incomer here, as even if the RCBO has failed you would have the back-up of the main incomer RCD to trip in the worst-case so no single point of failure.

But in practice the OSG is the worst-case: for lowest supply volts, highest magnetic trip tolerance, and 0.8 factor for hot cable resistance. If you take the average values you have 4 * 45A = 180A instant trip for a 45A B-curve MCB hence with a common 240V supply your Zs limit is actually 1.3 ohms if the fault happens on switch-on. Basically the risk is not quite so bad as initially appears, though for compliance with the regs you should be doing it worst-case or relying on RCD action (which is permitted, but something I kind of like to avoid as above).

This is all true, But i think the OP is talking about fault protection before the consumer unit and meeting discconection times of the main DNO fuse or downstream fuse/isolator etc

Which is why he mentioned it would need to be a 32a DNO fuse to meet disconnection time without using an RCD.

 

[01laverickd]

Here you are looking at the up-front S-type RCD due to the sub-main protection issue. If you only had the problem of a high Zs on a normal CU install then you are looking at either keeping the MCB trip points down, or depending on the RCD action of RCBOs.

Zs of 0.8 ohms is the upper limit usually assumed for TN-S, it can be more but i think something like 90% will be less. If they don't have any high current circuits you are OK even on the OCPD design, and what used to be done was to use a smaller fuse on any rings if you had a high feed Zs (e.g. 20A or 25A).

In theory your typical 30mA RCBOs are OK for Zs below 1667 ohms, but in reality you would not trust that as earth rods should be less than 200 ohms to be considered stable enough given seasonal changes in soil moisture and freeze risk.

So while the RCD side is your means of escaping from high Zs, it is still not really a good approach if you can meet disconnection on the MCB side as it is much simpler and more reliable than the electronics needed for the RCD function.

A quick look at the OSG table B6 shows that type B MCBs are OK for 0.8 ohms Zs up to 40A, then you start to have issues, so high power showers or cookers needing 45A or 50A supplies are your risk here depending on the RCD. Now you might decide to return to the 100mA incomer here, as even if the RCBO has failed you would have the back-up of the main incomer RCD to trip in the worst-case so no single point of failure.

But in practice the OSG is the worst-case: for lowest supply volts, highest magnetic trip tolerance, and 0.8 factor for hot cable resistance. If you take the average values you have 4 * 45A = 180A instant trip for a 45A B-curve MCB hence with a common 240V supply your Zs limit is actually 1.3 ohms if the fault happens on switch-on. Basically the risk is not quite so bad as initially appears, though for compliance with the regs you should be doing it worst-case or relying on RCD action (which is permitted, but something I kind of like to avoid as above).

A fantastic reply. Thanks very much. I really appreciate it.

"Now you might decide to return to the 100mA incomer here, as even if the RCBO has failed you would have the back-up of the main incomer RCD to trip in the worst-case so no single point of failure."

I agree with you are saying here but you wouldn't need the 100mA if you are fault protecting the circuit with an RCBO. But I know what your saying about if the RCBO failed and in any case it wouldn't offer additional protection but it would be better than nothing (as I'm sure the person on the bathroom floor would think aswell!)

 

[happysteve]

Just playing devils advocate here - and actually I think your idea of a 100mA S-type RCD is a good one in this case (just make sure you have double pole or switched neutral RCBOs if you do!), the time/current characteristics of the type of fuse I typically fit in a switch-fuse for a submain would be just about ok for a 60A fuse, assuming R1+R2 of your distribution circuit is low. Looks like a shade over 200A at 5s, which would give a max Zs of 1.09Ω at 70º (230 x 0.95 / 200).

The above taken from Bussmann datasheet number 720114 (new version available here, same graph).