I presume there's no DNO agreement that their fuse provides overload or fault protection to the heater circuit - so a clear BS 7671 non-compliance on that score.

Physically, it looks like it might just scrape though for L-N fault protection (0.793kA equates to 0.29Ω which just matches my old OSG for 16mm² and 100A BS 1361) - although changes to Cmin and whether the measurements were made with the conductors warm or cold could swing things either way. For L-PE we cannot say without more knowledge (is the c.p.c. also 16mm²?) - for larger fault currents RCDs alone may well not be fast enough to protect reduced c.s.a conductors).

Clearly a 100A doesn't normally protect 16mm² conductors from overload - but that then begs the question of whether overload protection is required. If it's a fixed load incapable of overloading, it may well not be required. Heating elements normally can't overload, but some internal faults (e.g. short part way along an element) can behave like overloads as far as the fixed wiring is concerned. A simple element in an earthed metallic jacket (like most mineral insulated elements) can't really short to another live conductor, or by-pass a section of element) (and if it shorted to the jacket, the RCDs should disconnect in this case). However a lot of higher power water heaters used uninsulated elements - so depending on how the elements are arranged and whether there's any internal overload protection, so again a bit more research needed.

   - Andy.

I presume there's no DNO agreement that their fuse provides overload or fault protection to the heater circuit - so a clear BS 7671 non-compliance on that score.

Physically, it looks like it might just scrape though for L-N fault protection (0.793kA equates to 0.29Ω which just matches my old OSG for 16mm² and 100A BS 1361) - although changes to Cmin and whether the measurements were made with the conductors warm or cold could swing things either way. For L-PE we cannot say without more knowledge (is the c.p.c. also 16mm²?) - for larger fault currents RCDs alone may well not be fast enough to protect reduced c.s.a conductors).

Clearly a 100A doesn't normally protect 16mm² conductors from overload - but that then begs the question of whether overload protection is required. If it's a fixed load incapable of overloading, it may well not be required. Heating elements normally can't overload, but some internal faults (e.g. short part way along an element) can behave like overloads as far as the fixed wiring is concerned. A simple element in an earthed metallic jacket (like most mineral insulated elements) can't really short to another live conductor, or by-pass a section of element) (and if it shorted to the jacket, the RCDs should disconnect in this case). However a lot of higher power water heaters used uninsulated elements - so depending on how the elements are arranged and whether there's any internal overload protection, so again a bit more research needed.

   - Andy.

Hi Andy. Thank you for your insights. An EICR has been conducted, and I am currently examining this specific certificate. This particular observation requires coding (I will speak with the inspector in the morning). Presently, there are no indications of thermal overloading; hence, in accordance with Regulation 433.3.1 (iv), overload protection may be deemed unnecessary due to the fixed load nature of the system, which should disconnect within five seconds under fault current conditions (referring to time/current graphs). However, as far as I am aware, this has not been agreed to with the DNO I seek yours and others perspective (I realise this is difficult without seeing the install but if you can using the info provided) on whether this should be documented as non-compliance with the Electricity Safety, Quality and Continuity Regulations (ESQCR). Would it be appropriate to classify it as (FI), a (C2) defect, or a (C3) defect?