Rcbo with higher than permitted zs

[Highspark24/7]

Hi I was testing a large holiday let property which has a three phase supply and dB
Now the original install was in 2012 and electrically is well done but poorly designs on socket circuits all rcbo are type c and only on socket circuits I am over the max permitted zs I would recommend change to type b but looking for some regulation to note on test cert

 

[pc1966]

You can disconnect on the RCD side of the RCBO to meet the regulations, as you generally have to for a TT system.

I would generally prefer to meet disconnection on the OCPD side of things on TN systems if at all practical, just as i believe the thermal-magnetic trip side is simpler and more reliable than the RCD electronics. But it is not an absolute must.

How far off meeting Zs are you?

Do you know if there are any other reasons for putting the sockets on C-curve breakers, such as it is used for motor loads with high start-up currents?

 

[westward10]

It is fine there is no need to change them an rcd can be used for fault protection.

 

[pc1966]

In terms of the regs, 411.4.5 states you can use an RCD to meet ADS.

411.4.204 covers the Zs requirement by citing Table 41.5 (which is basically 50V/In but not adding 200 ohm as max stable value)

 

[SparkySy]

How far over the max Zs are you and how does it compare to your R1+R2? It could be uplift caused by the RCD.
As said above by PC I also prefer not to rely on the RCD solely for fault protection but I it is acceptable.
Sy

 

[Highspark24/7]

You can disconnect on the RCD side of the RCBO to meet the regulations, as you generally have to for a TT system.

I would generally prefer to meet disconnection on the OCPD side of things on TN systems if at all practical, just as i believe the thermal-magnetic trip side is simpler and more reliable than the RCD electronics. But it is not an absolute must.

How far off meeting Zs are you?

Do you know if there are any other reasons for putting the sockets on C-curve breakers, such as it is used for motor loads with high start-up currents?

Thanks for the replay
Supply is a tncs with a ze of 0.15
At 250v
No just standerd domestic items realty on socket most socket circuits
Exsept garage has a small pump for the private water supply I think that's all that was available for rcbo at the time of install for the crabtree distribution board and looks like the electrician who designed the installation made poor choice of that equipment
Of the top of my head one socket circuit zs was 0.83

 

[Highspark24/7]

Thanks for the replay
Supply is a tncs with a ze of 0.15
At 250v
No just standerd domestic items realty on socket most socket circuits
Exsept garage has a small pump for the private water supply I think that's all that was available for rcbo at the time of install for the crabtree distribution board and looks like the electrician who designed the installation made poor choice of that equipment
Of the top of my head one socket circuit zs was 0.83

Also no rcd uplift I use a Megger 1721
And high quality crabtree rcbo from 2012

 

[pc1966]

Of the top of my head one socket circuit zs was 0.83

With Ze = 0.15 you have R1+R2 of around 0.68

Assuming 2.5mm T&E radial is 19.5 ohm/km then you are looking at around 35m length, so it a sane result, but in that case it ought to be on 20A which as maz Zs of 0.87 ohm so met?

If it was a typical 32A RFC on 2.5mm then r1+r2 = 4 * (R1+R2) so about 140m which is fairly long.

It is not big issue but may suggest something else is not quite right?

 

[Highspark24/7]

Also no rcd uplift I use a Megger 1721
And high quality crabtree rcbo from 2012

All dead R1 R2 an

With Ze = 0.15 you have R1+R2 of around 0.68

Assuming 2.5mm T&E radial is 19.5 ohm/km then you are looking at around 35m length, so it a sane result, but in that case it ought to be on 20A which as maz Zs of 0.87 ohm so met?

If it was a typical 32A RFC on 2.5mm then r1+r2 = 4 * (R1+R2) so about 140m which is fairly long.

It is not big issue but may suggest something else is not quite right?

It is a massive property and and every other test is consistent most of circuits would be fine on a 20 amp rcbo apart from kitchen i just think it was poorly designed in the first place. For example there is a 50 amp rcbo feeding 25mm swa to the annex / garage with a 14 way high integrity d.b with 4x rcbo and 2 X rcd and rest MCB

 

[westward10]

It probably was poorly designed but when it comes to testing it is what it is. Although not in your case a high Ze may leave you with no choice but to rely on an rcbo for fault protection I personally wouldn't look into it too deeply.

 

[pc1966]

Yes, not very good design but equally not dangerous.

The Zs is higher than UK regs on OCPD alone but given your 32A C-curve trip range for 230V covers Zs = 1.1*230/(5*32) = 1.58 ohms best-case down to 0.8 * 0.95 * 230 / (10 * 32) = 0.54 ohms worst-case, your measured 0.83 is going to be met for over half the tolerance range, and the RCD side always met.

 

[cliffed]

Agree, with the Rcd in circuit, R1+R2 tests, its compliant not ideal but ok.
The worry I have these days is that the lesser experience electricians, we’ve a high Zs reading put a Rcd on it.
Without the relevant tests it’s a major concern, days gone by you would certainly fix the problem

 

[pc1966]

Agree, with the Rcd in circuit, R1+R2 tests, its compliant not ideal but ok.
The worry I have these days is that the lesser experience electricians, we’ve a high Zs reading put a Rcd on it.
Without the relevant tests it’s a major concern, days gone by you would certainly fix the problem

Exactly, if it is just a rather long cable then it is not so serious, even though it is probably not meeting the VD spec either. But it could be something more serious that ought to be fixed (bad joint, etc) and not simply side-stepped with the RCD aspect.

 

[Highspark24/7]

Thank you all so much for the replies
And sorry for late response I'm working away at the moment and haven't had much time or that good internet connection I had noted on cert that it was a poor design and there is a few other issues I am due to go back and rectify once I am working back local thanks again the collective hive mind