First CU change - high R1+R2 reading and other problems!

[malcolmsanford]

Your maximum Zs value for a 30mA RCD is given in table 41.5 of the BRB. As your fitting to the 17th edition your sockets in the home will come under several regs those being 411.3.3 and 522.6.6 and therefore be rcd protected.

As you know your touch voltage for fault protection must be <= 50 V so therefore 50/0.03 is 1666.66 recuring hence 1667

 

[Deleted member 19807]

ah yes 1.67, i knew it did not sound right, thanks for the correct figure. the 1666 ohms is to limit for a 30ma rcd to achive a touch voltage to <50v in the case of a fault. I still like my reading to be close to what i think thay should be as it indicates to me a latent fault somewhere that may get worse if not tracked down.

 

[Marcosparco]

Thanks a lot Malcolmsanford - for some reason I'd thought that table only applied to TT systems previously, so hadn't realised that RCD's benefited the Zs on TNS so much! So while r2 and thus r1+r2 is high, because it comes under 1667 ohms and disconection times are met, am I right in saying it is acceptable? Regarding completing the EIC, if I understand the logic despite measuring a max Zs of 1.22 ohms the theoretical max is 1667 ohms and that therefore should be what is recorded?

 

[Smugley]

I'm not comfortable about recording 1667 as the Max Zs for each circuit just because it's protected by an RCD. I would prefer to put down the MCB's Max Zs because it gives you the information necessary to confirm that your R1+R2 and measured Zs values comply with table 41 for overcurrent and overload and clearly shows that you have checked this. You can always put 1667 down in your test results next to the RCD. I'd be interested to know if someone has a good reason for not doing this that I may not have considered.

 

[IQ Electrical]

I'm not comfortable about recording 1667 as the Max Zs for each circuit just because it's protected by an RCD. I would prefer to put down the MCB's Max Zs because it gives you the information necessary to confirm that your R1+R2 and measured Zs values comply with table 41 for overcurrent and overload and clearly shows that you have checked this. You can always put 1667 down in your test results next to the RCD. I'd be interested to know if someone has a good reason for not doing this that I may not have considered.

Leaving your comfort to one side The advice given by Malcolm is absolutely correct, the max Zs to be recorded for a BS61009/BS4293 RCBO device is 1667 Ohms. Automatic disconnection is achieved by means of the RCD.
Once continuity of protective conductors is confirmed, the Zs is almost immaterial.

 

[Marcosparco]

Thanks both - Update....I wanted to improve the r1+r2 , but also wanted to make sure I was following the logic correctly (and filling in the EIC correctly). As there was only one ring for the whole house, I;ve split this into 2 radials on 20amp mcb's. Logic that it spreads the load better, plus both radials come in at a Zs of under 1.84ohms rather than struggling on the ring to come under 1.15ohms for the 32amp mcb.

Hopefully that makes sense and is good practice?

It's amazing how the real world of old circuits etc is different to the clean environment of college - ok not that amazing really! While frustrating working alone and scratching my head when it is not getting the expected results, I think I'm learning a lot more now than on boards at college - plus getting some words of wisdom from those of you who've seen it all before. Off to tidy up paperwork and to book in for the assesment, hopefully for before xmas - thanks again.

 

[malcolmsanford]

And as soon as RCD's are used everywhere as on the continent the bonding regulations will also be relaxed, perhaps not my lifetime in this industry but certainly within the next few admendments of the regs.

We have already started with supplementary bonding in locations with a bath and shower, and as soon as RCD's become as wide spread in industrial /commercial applications as they now are in domestic bonding will be drastically reduced.

I think the NICEIC have already started to pave the way by issuing an article about if you can prove that their is less than 7Mohm between metal parts then bonding is irrelevant, I can't find this at the moment, and if I'm wrong I would gladly be put right by someone.

 

[IQ Electrical]

Guidance note 8 but I think it's 23K Ohm

 

[IQ Electrical]

Ok, if the resistance of the metallic part in question to the main earth terminal exceeds 23K Ohms then it is not deemed to be an extraneous part and will not require supplementary bonding.

I'm going from memory here as my digital trial of Guidance Note 8 ended this morning and I'm 'out and about'.

 

[malcolmsanford]

Pretty impressive IQ none the less .......................

 

[IQ Electrical]

Pretty impressive IQ none the less .......................

Ha, until I get back and check GN8 to find I'm a mile out