- Sep 24, 2013
- 1,519
- 2,493
- 3,688
- If you're a qualified, trainee, or retired electrician - Which country is it that your work will be / is / was aimed at?
- United Kingdom
- What type of forum member are you?
- Practising Electrician (Qualified - Domestic or Commercial etc)
- Business Name
- Dovecote Electrical
Just out of interest, what make/model of earth fault loop tester (presumably MFT) are you using? Some makes are better than others at no-trip Zs measurements through RCDs - on some you get "RCD uplift" of an ohm or so. I suspect the higher the Ze - particularly if it's variable - the more likely you'll get a discrepancy. Using the alternative method of measuring Zs, when there is an RCD present, is often the only way to get a reliable reading.
My concern about your high Ze is that it might be indicitive of a developing fault, one that's getting worse. It might be 1.14Ω this week, but if it's symptomatic of a breakdown in the lead sheath of the supply cable, it might well suddenly go much, much higher. You'll then be relying on main protective bonding for your RCD to work (assuming you've got metal pipes/anything else extraneous).
The DNO are within their rights not to supply a means of earthing if they don't want to. But I'm very surprised at their answer which seems to effectively be, "Yes, we're supplying an earth, but it's not a very good one, so bung an RCD on it."
My concern about your high Ze is that it might be indicitive of a developing fault, one that's getting worse. It might be 1.14Ω this week, but if it's symptomatic of a breakdown in the lead sheath of the supply cable, it might well suddenly go much, much higher. You'll then be relying on main protective bonding for your RCD to work (assuming you've got metal pipes/anything else extraneous).
The DNO are within their rights not to supply a means of earthing if they don't want to. But I'm very surprised at their answer which seems to effectively be, "Yes, we're supplying an earth, but it's not a very good one, so bung an RCD on it."
