High Ze causing headache !

[sythai]

Hi Guys

Any advice/ pointers would be appreciated

Have ‘pretty big’ new build house we’re wiring from scratch.

New supply in from DNO is PME, but we have Ze of 0.51

Are DNO obliged to keep this under 0.35?

Wouldn’t be that much of an issue normally if regular size house and meter was attached to house.

Before we came along on the scene Client had separate garage block built, meter permanently sited in here.

We’ve run a SWA sub-main across to house DB 25metres away, but struggling on suitable overcurrent protection. Even at 80amps BS 88…. Max EFLI 0.40!

Don’t really want to be putting new build on TT system, if it comes to it.

Thank you
Sy

 

[TJ Anderson]

My bad, it isn't only concerned with protective conductors:

434.5.2
A fault occurring at any point in a circuit shall be interrupted within a time such that the fault current does not cause the permitted limiting temperature of any conductor or cable to be exceeded.

Nice one, that's the reg and the one that often gets fell foul of.

 

[westward10]

Even changing to TT, I assume at

Fill it in with the correct values, but in notes/remarks add something like "ads achieved via rcd as fault protection " - according to each circuit that needs it, for example lighting circuits probably achieve ads with the mcb so don't need it.

Then in 3.1 the last line rcd as fault protection should be ticked (and ideally the appropriate circuit number added - so rfc downstairs, shower... whatever).

You would still need to tick rcd as additional protection as a house requires it for lighting, sockets etc anyhow

I wouldn't add a note, BS7671 accepts an rcd can be used for fault protection.

 

[Julie.]

Even changing to TT, I assume at

I wouldn't add a note, BS7671 accepts an rcd can be used for fault protection.

I would add a note, as you say it isn't required, but it provides clarity for anyone that looks at it in the future.

 

[westward10]

I know a lot of people feel designing a circuit on a TN system and relying on a rcd for fault protection is a bad design but this is twaddle and this thread is a good example, a high Ze but even so it is still acceptable.

 

[David Prosser]

No chance of using a BS88-2 63A fuse? I've have access to three different MFT, never got a matching Ze reading from any combination of them. One (Fluke) is always higher than the other two (Megger) with it not unusual to see a difference of up to 0.2 between the Meggers and the Fluke.

Be interested to see what the Zs is when the installation is completed as apposed to the Ze.

 

[sythai]

No chance of using a BS88-2 63A fuse?

Afraid not.... fully loaded: ASHP, 32a Induction Hob, 2 x ovens etc
and 4000sq ft so pretty big place!

 

[sythai]

I've have access to three different MFT, never got a matching Ze reading from any combination of them. One (Fluke) is always higher than the other two (Megger) with it not unusual to see a difference of up to 0.2 between the Meggers and the Fluke.

I am going to try our other ones... we've got couple of other Meggers - be interesting to see

 

[pc1966]

It was on the IET forum about why we should not use adiabatic for fault currents operating devices <0.1S or >5S. I couldn't remember but just found it again on there.

Going over 5S was more straightforward. Because the adiabatic is simplified it does not allow for heat loss, this results in artificially high csa's

The 5s is somewhat arbitrary and related to the ADS regs. In reality the issue is as you say, the CSA becomes larger than necessary as it is not accounting for heat loss (i.e. it is not really adiabatic). So it is a safe option, but a sanity check is if the computed CSA is larger than the necessary line conductor to match the CCC limit of the OCPD then you really have gone beyond the limit!

Going under 0.1s was more complex. 0.1S is 5 cycles of the supply waveform and going below that could give rise to greater currents than calculated due to the waveform being asymmetric and becoming distorted.

Sort of. What you see in the time/current plots is typically known as "virtual time" and it is the equivalent time at that PFC leading to the same I2t let-through. In reality, and especially for fuses that are quite energy-limiting, the time on the plot can be uses with tolerable accuracy even allowing for varying phase angle of the fault.

So while you might see an extraordinary short time like 10-100us listed at high PFCs, in reality the fuse arcs for longer but is limiting the current so you don't see anything like the PFC flowing. That is also seen on the plots of peak fault current where it is much lower than you get from MCB/MCCB.

 

[pc1966]

I know a lot of people feel designing a circuit on a TN system and relying on a rcd for fault protection is a bad design but this is twaddle and this thread is a good example, a high Ze but even so it is still acceptable.

I would normally avoid relying on RCD if reasonably possible, but as you say where the only sane technical solution is RCD then it is perfectly good.

Same for the earlier comment about going TT, it is much better in most cases to have the low TN supply Ze used, even if it is not quite low enough for the sub-main, as it means the final circuits are likely to disconnect on OCPD alone which is a small extra reliability gain, as well as having lower touch potential during any fault.

 

[pc1966]

New supply in from DNO is PME, but we have Ze of 0.51

We’ve run a SWA sub-main across to house DB 25metres away, but struggling on suitable overcurrent protection. Even at 80amps BS 88…. Max EFLI 0.40!

Actually there is something more fundamentally wrong here. If it is TN-C-S then, unless there is something very bad at the cut out, the supply Ze is the same as the supply impedance. With 0.51 ohms and 80A you would be seeing 40.5V drop in the incoming supply! That is more than the ESQCR limits of +10% -6% on 230V

Are you sure it is actually rated above 60A anyway? I think more discussion with the DNO is needed.

 

[Lucien Nunes]

On a TN-C-S service where the L-N and L-E loop impedances are equal, surely Zs = 0.51Ω implies there will also be a problem for voltage drop?

Wow, exact simulpost with @pc1966!

 

[pc1966]

Wow, exact simulpost with @pc1966!

I'm just jealous you have a way of typing Omega in text!

 

[sythai]

Actually there is something more fundamentally wrong here. If it is TN-C-S then, unless there is something very bad at the cut out, the supply Ze is the same as the supply impedance. With 0.51 ohms and 80A you would be seeing 40.5V drop in the incoming supply! That is more than the ESQCR limits of +10% -6% on 230V

Are you sure it is actually rated above 60A anyway? I think more discussion with the DNO is needed.

Ok so I may have a case then..... assuming DNO have to comply with ESQCR? (not something I'm familiar with being honest)

Its got to be above 60a (for size of property) going to get the Client to dig out the original paperwork from when had it installed.

 

[buzzlightyear]

Are you sure it is actually rated above 60A anyway? I think more discussion with the DNO is needed.

That what I said earlier confume with them before you do any thing else, before you take another route.

 

[timhoward]

As a sanity check, have you taken a L-N reading?
Could this all be a loose main earth connection in the cut-out?!