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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
Are the resistances you see compatible with the size of the building and likely wiring lengths ?
(each core of 1mm cross section is about 16 milliohms per metre long, so 3 ohms sounds like you have hundred meters or so of cable, perhaps 1600milliohms to get to the the fitting and the same again coming back )- sounds like rather a lot of wire if not and it is a small flat then there is a connection problem )
and what is the Zs at the board as it were ? (always possible the fault is really upstream, and was not there when it was put in, and now the bonding has come off or something.)
Something is not really right and a bit more info is needed.
Mike, Zs at DB is 0.12 so pretty low. They are long corridors with approx 24 lights/sensors (hence joints) along the run, also an additional rise/drop of up to 2 floors through the riser so figures look feasible. Tested live with high current loop test, got slightly higher (worse) using r1r2 dead test. This is obviously communal areas btw not an individual flat
Alan, of course you are right, 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. Suppose the issue would be someone cutting or damaging the cable.
Hmm sounds like it has been marginal design right from the off. Anyone using more than one hundred metre reel on an installation really should check their loop impedances quite carefully.
A C10 might at the limit need 100A to instant trip, so your loop impedance should really be comfortably under 2.3 ohms with a suitable margin for low supply voltage (your 2,19 figure sound about right.).
I think you are looking at a C2, and the recommended fixes in order of ease and practicality will be either change to RCBO protection, derating to a lower current breaker or linking CPCs and or adding something to lower the R2.
I get the same thing quite often C curve MCBs in EICR’s i do in communal flat areas, if there feeding LED lights i change them to type B which has double the maximum ZS.
