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Zs readings over limits
Former Community Member
Doing an EICR in a block of flats and quite a few of the lighting circuits have excessive Zs readings. All the circuits are on 60898 C10's without RCD protection with readings in the late 2 early 3 ohm range (max permitted 2.19). I know test result fails are generally a code 2 but these are LED panel downlighters with mostly class 2 drivers connected via choc boxes above the suspended ceiling so nothing really accessible. They would all comply with a C6 btw. Just wondered if you'd C2 this resulting in an unsatisfactory EICR
prob a c6 would be better than a b10 considering surge current nuisance tripping (60A vs 50A)
But MCBs are liable to instantaneously trip anywhere within their multiplier range - 3-5x for a B type or 5-10x for a C-type - thus you need to stay below the lower multiplier when you want an MCB not to trip - hence 30A for a B10 (10x3) or 30A for a C6 (6x5).
At 3ohms you are likely to get around 70 to 80A and push disconnection to around 6 to 7 seconds. Providing I2t is ok then the risk is minimal for what you describe.
under fault conditions ADS has to work to prevent anyone getting an electric shock but what would you code a standard class 2 lighting pendent that had no earth connected? there are no exposed conductive parts on the circuit to get a shock from, the Zs test was made on the unterminated cpc of the T&E cable at the end of the circuit.
I can see the point that it could (in theory) be declared a double/reinforced insulation circuit - and the c.p.c. omitted altogether and Zs would be irrelevant (as it's in a common area, rather than domestic, the bit about ensuring adequate supervision could perhaps be complied with).
With a c.p.c. present however there would seem to be a risk of a L-c.p.c. fault (especially with T&E where the c.p.c. isn't insulated as if it were a live part), which although wouldn't make any exposed-conductive-parts on that circuit live (as there aren't any) - it would make the c.p.c. itself hazardous live - and so too anything connected to it - e.g. exposed-conductive-parts of other circuits connected to the same DB for instance. Granted the touch voltage on remote parts would likely be much lower than at the fault - but if the disconnection time is really extended that still might not be sufficient to ensure safety.
Andy, thats along the line I was thinking. Do remember if that fault was realised we would still get disconnection, just not in 0.4 seconds, as the thermal part of the MCB could come into play. Same with P/N impedance mentioned by Alan earlier, I dont think that applies as again we are in overload territory. As I see it exposed live conductive parts = 0.4s anything else = protect the cable.
I will code it C2 btw but cant help thinking its pretty safe as it is.
It seems to me that this circuit is non-compliant, but is relatively low risk as parts connected to the CPC can not be readily touched.
To rectify, several approaches are possible.
Fit a smaller MCB*, fine WRT to load current but possibility of tripping on inrush current.
Fit a faster acting MCB*, also chance of tripping on inrush currents.
Rewire in larger cable to reduce the loop impedance, arguably the best solution but expensive and disruptive.
Install an additional CPC, also costly and disruptive.
Connect the CPC of this circuit with that of another nearby circuit. Complicates future testing, and could be rendered ineffective by future changes.
Fit an RCD, preferably not a 30ma type due to risk nuisance trips. A 100 ma type would give quick acting protection with the reported loop impedance.
*Be aware that a lower rated or faster acting MCB might have been the original design, and that the presently fitted device MIGHT have been retrofitted when the LED lights tripped the original OCPD.
... but what would you code a standard class 2 lighting pendent that had no earth connected? there are no exposed conductive parts on the circuit to get a shock from ...
This topic came up this afternoon when my self-building chum and I discussed the apparent futility of terminating the CPC in the otherwise empty terminal in a ceiling rose with a standard pendant fitting (2-way, so not looped at the rose). Leaving aside the requirement in Bs 7671 to provide a CPC, there is always the possibility that the fitting may be replaced by a class 1 one.
In answer to the above, if class 2 fitted and CPC terminated correctly => no code; if class 2 fitted and CPC snipped off => C3 (couldn't be worse because no fault to danger); if class 1 fitted and CPC snipped off => C2.
