what are your thoughts guys

[chrish_1977]

Done an eicr on a 2 bed flat, all good apart from the sockets, it's a radial, wired in 2.5 t+e (25 sockets in total), on a 20 mcb, zs was 2.72 ohms, I've checked against bgb and its over the maximum permitted zs, customer now wants it sorted, but I'm wondering what you guys would think is the best way to go about sorting this out. Choices I can see are, splitting the radial into two, or splitting into two and making the longest leg a radial.
The whole flat has just finished being decorated and all carpets down etc

 

[silverfox1]

These 2 question / answers are from the Electrical Safety Councils web pages .

Q1.29
As the designer of an installation, am I allowed to rely on the RCD element of an RCBO to provide for fault protection in order to allow for loop impedance values greater than given in Table 41.3?
Yes, so long as all the other applicable requirements of the 17th Edition (as amended) are met, including those for protection against overload and short circuit.
Regulation number(s)

• 411.4.4
• 411.4.5
• 411.4.9

Q1.35
Where the earth fault loop impedance for a circuit exceeds the maximum Zs for the overcurrent device, is it permissible to use an RCD?
Yes.
Regulation number(s)

• 411.4.4
• 411.5.2

 

[chrish_1977]

Thanks silverfox, but I am not the designer, so in my eyes it's not ok, this circuit from what I can see has been added too over the years, hence the number of points. When I get a chance will look through the bgb and go from there

 

[jamie-spark]

These 2 question / answers are from the Electrical Safety Councils web pages .

Q1.29
As the designer of an installation, am I allowed to rely on the RCD element of an RCBO to provide for fault protection in order to allow for loop impedance values greater than given in Table 41.3?
Yes, so long as all the other applicable requirements of the 17th Edition (as amended) are met, including those for protection against overload and short circuit.
Regulation number(s)

• 411.4.4
• 411.4.5
• 411.4.9

Q1.35
Where the earth fault loop impedance for a circuit exceeds the maximum Zs for the overcurrent device, is it permissible to use an RCD?
Yes.
Regulation number(s)

• 411.4.4
• 411.5.2

Yes agree

I think if you have ruled everything out like slack connections ect ect. where circuit resistance with ze make for zs slightly higher than that prescribed. Then mark down max zs on cert for the circuit as 1667 and move on. I think what is wrong is people who "design" circuits then find out zs is to high and just slap an rcd in cos it covers it and don't know any reasons why and if it will be ok under short circuit conditions

 

[shagbite]

Q1.29
As the designer of an installation, am I allowed to rely on the RCD element of an RCBO to provide for fault protection in order to allow for loop impedance values greater than given in Table 41.3?
Yes, so long as all the other applicable requirements of the 17th Edition (as amended) are met, including those for protection against overload and short circuit.
Regulation number(s)

• 411.4.4
• 411.4.5
• 411.4.9

or, if it's TN, you could just design the installation properly in the first place.

 

[jamie-spark]

[/LIST]


or, if it's TN, you could just design the installation properly in the first place.

Agree with that too. How many hoose bashers just bang 1.5/ 2.5 / 6mm in and on a good day maybe some 10mm without any calcs in first place

 

[Plonker 3]

Any spark worth his salt will design a circuit correctly, if it isn't then are not a spark simple as that.

 

[davethsparky]

I think we all agree that it should not have been designed this way, but the question is not about design.

The question is about what to do now that this problem has been found in an existing install which has been newly decorated and has new carpets fitted. I think it goes without saying that the design of the circuit is wrong, but everyone jumping in and telling us all how bad the design is can't be helping with solving the original problem.

The way I have read this is that the customer is going to refuse to have new cabling put in as it will disturb the decorations and a 16A MCB may be inadequate.
If the circuit can be reduced to a 16A MCB then great, but if it cannot and if the customer wont pay for new wiring to be put in for whatever reason what do you do? Do you just sit there complaining about the poor design of the circuit or do you the best you can to improve a bad situation?

RCD protection is not the right answer to the problem but it is an improvement to the situation.

 

[shagbite]

I think we all agree that it should not have been designed this way, but the question is not about design.

The question is about what to do now that this problem has been found in an existing install which has been newly decorated and has new carpets fitted. I think it goes without saying that the design of the circuit is wrong, but everyone jumping in and telling us all how bad the design is can't be helping with solving the original problem.

The way I have read this is that the customer is going to refuse to have new cabling put in as it will disturb the decorations and a 16A MCB may be inadequate.
If the circuit can be reduced to a 16A MCB then great, but if it cannot and if the customer wont pay for new wiring to be put in for whatever reason what do you do? Do you just sit there complaining about the poor design of the circuit or do you the best you can to improve a bad situation?

RCD protection is not the right answer to the problem but it is an improvement to the situation.

firstly, i'd go with dillb's solution and drop the breaker to a 16A.

other than that, if the customer wants me to make her installation safe, she has to pay for a new circuit. if she wants to put all her faith in the reliability of a RCD, then she can get someone else in to fit it.

 

[telectrix]

if this were a TT system with 30mA RCD protection, you'd not give it a second thought. although poorly designed/modified,if there is RCD installed, it's just a C3 on the cert. and recommendation of improvement.

 

[shagbite]

if i was just doing a report tel, i'd fully agree. but if i'm charged with making the installation safe, then i'd insist on a new circuit.

 

[chrish_1977]

I had been asked to test and give an all ok, but upon finding this I'm reluctant to say "yeah it's all good" when it's clearly not. So even though its above the max for the 20a mcb it's still a c3 I would have said maybe c2, and because if this I have stated it unsatisfactory. Would it be better to make the cert satisfactory and just note that the offending circuit has a very high zs but has rcd protection, but there may be a question as to whether the cpd will operate within the prescribed times?

 

[Sharpend]

Now here's a thought, could you not put this circuit on its own 1361 fuse breaker as opposed to an MCB, isn't the Zs allowed to be higher on a 1361? Or am I getting confused, I'm just trying to think whilst having a spot of lunch and don't have th BGB to hand.

 

[Sharpend]

Just run down and checked a 15a 1361 has max Zs of 3.28 for 0.4 disconnection?

 

[Risteard]

Is it just me, or have we not been informed what the Ze is?

 

[telectrix]

I had been asked to test and give an all ok, but upon finding this I'm reluctant to say "yeah it's all good" when it's clearly not. So even though its above the max for the 20a mcb it's still a c3 I would have said maybe c2, and because if this I have stated it unsatisfactory. Would it be better to make the cert satisfactory and just note that the offending circuit has a very high zs but has rcd protection, but there may be a question as to whether the cpd will operate within the prescribed times?

i'd argue with the C2 as , IMO, there's not a potential danger. that 20A is going to break pretty sharpish @82A PFC. and the RCD within 40mSec. anyway. i'd also question the 0.8 factor in this situation, as there's normally no way that the cable will be operating at or close to it's 70deg. limit.