Been out to replace an oven on an installation which pre dates the need for RCD's so only has MCB's, the earthing arrangement is TT. As I am only changing the oven this need not be RCD protected but is the maximum Ze still 200ohms?
- Thread starter Jon Bridges
- Start date
D
Deleted member 26818
The maximum Ze allowed is one that will allow any device installed with the intention to provide fault protection, to operate.
With a 30mA RCD, that would be 1667 ohms.
With a 30mA RCD, that would be 1667 ohms.
Understood, however this installation does not have an RCD and the work I am carrying out, ie replacing a cooker, does not require an RCD. I need to know the max Ze for a TT system without an RCD.
Thanks
Thanks
D
davesparks
You need to calculate it as it will depend entirely on the specifics of the installation. But if you take the circuit with the lowest maximum permitted Zs and subtract the R1+R2 of that circuit from it then it will give you what is theoretically the maximum Ze that will comply.
As a general rule of thumb the Ze will need to be stable below 1 ohm to allow the use of mcbs as fault protection.
The 200ohms figure you mentioned in your op is a 'suggested value' above which Ra may not be stable, it is not the maximum permitted for an earth electrode. The maximum acceptable value is one which is reliably stable and fulfills the requirements of the protective measures put in place and will vary depending on each installation and he soil conditions around it.
Thanks Dave. That makes sense. I have never seen a TT Ze lower than around 20ohms so am I to assume the install was incorrect when it was fitted approx 30 years ago?
D
davesparks
No you are not to assume anything, you test and confirm that an installation is safe, preferably before you start to work on it otherwise the customer could reasonably refuse to pay for extra work you need to do to carry out your job safely if you have not made them aware of the costs before you start.
I certainly would not assume that a 30 year old earth electrode is still as good as the day it was fitted, or even if it was tested at the time. At that age a trip of some description, either VOELCB or ELCB, would have been required to be fitted by the regs so if it is not present it would suggest that the regs were not adhered to in the first place
D
Deleted member 26818
Dilema here, is that you are being asked to install an item of equipment to a circuit with no fault protection.
If you consider this work to be within the scope of BS7671, then you will have to provide fault protection.
If you consider this work to be within the scope of BS7671, then you will have to provide fault protection.
D
davesparks
Sorry I misread your post.
O
Octopus
Hum... Replacing an oven. Presumably just disconnecting the old one from the cable and fitting the new one.
i would do a mwc, pointing out the lack of RCD protection and submit it with my invoice.
i would do a mwc, pointing out the lack of RCD protection and submit it with my invoice.
How can you certify it if it is dangerous and non-compliant?
O
Octopus
Who said anything about certifying anything safe?
That's what the certificate indicates though. You are certifying that it complies with BS 7671 (and is safe).
O
Octopus
I have to disagree with you on this. The very should specifically say that the circuit does NOT comply.
It says clearly that "I certify... that the electrical installation work is in accordance with BS 7671:2008 as amended to 2015".
O
Octopus
"except as detailed in Part 1 above"
And in part 1 you add the details. There is the section in Part 2 "comments".
At the end of the day what should a prefessional do?
1. Issue a MWC with notes in Part 1 and Part 2
or
2. Not issue a certificate?
I would rather do 1 above!
If you are saying that ALL certificates state 100% compliance with the regs we better all give up now. Issuing a MWC with a note and comment about a potential fault is the better course of action IMHO.
Just asking for money and ignoring the issue makes us no better than bob from down the pub!
The correct answer is that you should remove the dangerous non-compliance before carrying out the work and certifying it. What you are connecting up is wholly reliant on the fault protection being present for safety, so it cannot be ignored. Likewise we are required to ensure that earthing and bonding are satisfactory before carrying out works.
O
Octopus
I refer you to 1 above. If the people have had a cooker connected in such away for 35 years, they may not accept your view that its not safe and needs improving.
The regs and rules are MAD IMHO.
So you would leave them without a cooker?
Then bob from the pub connects the cooker?
I will continue doing what I do....... a MWC, stating that the circuit doesn't comply is my record of flagging it to the customer...
O
Octopus
hum. You are going to need a very big pack if you take that approsch
Perhaps, but you couldn't possibly Certify an installation which has no earth fault protection.
D
Deleted member 26818
Does the conection of a cooker fall within the scope of BS7671?
The fact that you would issue an MWEIC, suggests that it does.
Does either the cooker or any cables used to make the connection require ADS?
If either do require ADS, then if it is not already there, it must be provided.
The fact that you would issue an MWEIC, suggests that it does.
Does either the cooker or any cables used to make the connection require ADS?
If either do require ADS, then if it is not already there, it must be provided.
My two pennith, would it not be the Zs that is to be considered?
Reg 411.3.2.4 TT system; each distribution circuit disconnection time not exceeding 1 sec.
Then reg 411.5.4 would give the calculation to give the maximum ZS when using a OCPD giving disconnection time of 1 sec;
Zs x Ia (less than or equal to) Uo x Cmin?
Reg 411.3.2.4 TT system; each distribution circuit disconnection time not exceeding 1 sec.
Then reg 411.5.4 would give the calculation to give the maximum ZS when using a OCPD giving disconnection time of 1 sec;
Zs x Ia (less than or equal to) Uo x Cmin?
L
Leesparkykent
Its a final circuit though. If disconnection is achieved by an OCPD and 411.3.1.2 is adhered to then the maximum disconnection times applicable to a TN system may be used if the circuit doesn't exceed 32A.
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Ok stand corrected.
PS but does not reg 411.5.4 apply to calculate max Zs when using ocpd on TT?
Last edited:
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