Measuring R1+R2 but unable to measure Zs

[ringer]

Am testing a lot of tubular heater circuits in a church. All circuits supplied with MICC, which runs to SFUs then via short length of heat-resistant flex to the tubular heaters. Have tested approx 40 SFUs/heaters so far without problem - R1+R2 (dead), IR (dead), then Zs (live) on each one. However, there is one particular SFU which is causing problems. There are eleven SFUs on this circuit, ten measure out okay, just this one doesn't. I measured R1+R2 (linked at the board, measured at the SFU) and got a reading. Livened the circuit to measure Zs, but tester will not take a measurement. The heater is working - I know, because it is getting hot! There is 240V L-->N at the SFU, so the heater warms up. However, there is no voltage L-->E, so the test instrument (Megger 1552) will not take a reading.

Have checked that instrument probes are definitely touching the terminals. How is it possible that I can get a R1+R2 measurement, the heater works when powered, but there is no voltage L-->E?

Can not get my head around this one - lots of other circuits/SFUs measure out ok, just this one that doesn't. Have repeated the R1+R2 measurement just to make sure.

Left circuits off for moment until I get back to test some more.

Any ideas?

 

[Boatelec]

it sounds like the heater being on that's messing it up. The circuit needs to be powered up obviously but all consumers need to be disconnected. I assume the tester "sees" the volt drop across the heater. It's usually a mobile 'phone charger that buggers my Martindale up. Howard

 

[UNG]

Where is the SFU that is causing the problem located in the circuit. Have you checked for a dodgy piece of MI or a dodgy pot seal

 

[sedgy34]

initial thought!!!! maybe a faulty terminal inside the heater but enough to get heater working???

 

[trebor]

I dont want to insult you but as you have not mentioned it, you have I presume checked with a voltage tester to rule out any dodgy connections and are you testing on a pot flylead or on the sheath or both, as the heater is working it must an earth problem

 

[tigerpaul]

Strange how you have continuity along the line and earth conductors for r1+r2, yet it is lost when the circuit is live.

Double check the earth has been put away in the board, maybe it's fallen out or you've forgot to put it back, **** happens and we've all done it.

 

[Adam]

Have you proved that the "live" conductor you used for the R1+R2 is actually the live conductor, and not neutral?You will still get a reading if the neutral is still connected at the DB.Try testing the voltage between neutral and earth. You may find this is 230v.

 

[ringer]

Think you may be right there, Adam. Anyway, I have switched off the circuit and said it requires further investigation. It is a heating circuit after all, and just for one set of pews - its not like they are going to need to use it anytime soon. Reversed polarity - such a silly mistake, yet it had me scratching my head for hours. I was convinced there must be two faults on the circuit - it did not occur to me that with the neutral still in the rail I would get a reading. Have yet to prove this is the fault, but am pretty sure now it is reversed polarity. Will have to chalk this up as 'experience' and eat the time wasted on it.

 

[nickblake]

If you linked the live conductor at the board and got a reading at the live of the FCU when the FCU is switched off then the polarity wouldnt be the wrong way round ,think you problem could well be a poor connection at the FCU that is between the terminals and the prods

 

[Adam]

If you linked the live conductor at the board and got a reading at the live of the FCU when the FCU is switched off then the polarity wouldnt be the wrong way round ,think you problem could well be a poor connection at the FCU that is between the terminals and the prods

Only if the Live at the FCU is the actual live. It is an MI, so OP would of relied on L markings in FCU or coloured sleeving / tape. If the Neutral was connected to the live terminal the R1+R2 would of tested the Neutral / Earth and been an expected value.

 

[steveberry11]

I think you need to check the polarity at the FCU. You will not get a Zs across earth and neutral. Easy wiring fault on MI

 

[londonlec]

The 1553 bleeps when polarity is wrong, but only when you use the neutral lead too I think?

I know that no RCD = no neutral lead though.

 

[nickblake]

Only if the Live at the FCU is the actual live. It is an MI, so OP would of relied on L markings in FCU or coloured sleeving / tape. If the Neutral was connected to the live terminal the R1+R2 would of tested the Neutral / Earth and been an expected value.

Totally agree Adam but as he got a reading on r1r2 then he should get a Zs reading if he connected his test leads to the same terminals which i assume he checked were the Live ones , if he carried out his r1r2 and the polarity was reversed then he wouldnt have got an r1r2 reading