120V on CPC and annoying MFT1552 fault

Joined
11 Apr 2011
Messages
166
Reaction score
14
Location
Tyne and Wear
Country
United Kingdom
Hi,
I recently rewired a lighting circuit, very simple set up of 5 pendants, 2 batten holders and 1 class I metal chandelier type fitting.

Install is TT with Ze 62 ohms. IR all spot on and R2 approx 0.52ohms.

I discovered that the CPC had 120V when it was not connected to earth. I know about inductance but the 120V seems a lil on the high side. The only apparent source of this increase IMO is from 2 of the switch drops which run for approx. 1.2m (each) through old steel conduit to the switches. There are no old cables in these drops, and the 1.2m conduits are not connected to anything else.

Also, my MFT1552 occasionally comes up with the fuse blown indicator whenever I try to check continuity or take an r2 reading. I have to take the fuse out and re-insert it for it to work again. Bl00dy annoying it is.

Does anyone have similar experience with these Meggers? Or even answers maybe..! I have not yet contacted Megger as I guess their stock answer will be it requires looking at and will cost a megger amount of dough!

Merry Xmas
 
Sponsored Links
Yep an Avo is a good bet. We've seen similar volts on a dead cable cores that when other means were used disappeared!!
Basically a cable had been made dead for safety, jointer's test lamp was broken so he used a digital meter and got similar readings, further investigation with other means showed it was an erroneous reading
 
Sponsored Links
Yep an Avo is a good bet. We've seen similar volts on a dead cable cores that when other means were used disappeared!!
Basically a cable had been made dead for safety, jointer's test lamp was broken so he used a digital meter and got similar readings, further investigation with other means showed it was an erroneous reading
 
Thanks again.

Will purchase one and give that a go. I tested it also with a Fluke Volt Indicator; that showed up no voltage, and with a digi multimeter (£30 jobby); that showed approx 112V.

Any other sober/drunk input welcome! Especially on the MFT...
 
You can get meters with a low impedance input for this very reason. Fluke 117 is a good example, even has a non-contact detector in it.
 
Thanks Monkeh,

I guess I am after a true RMS type of meter or an analogue one then. That fluke you mentioned looks pretty decent. Might wait and see if ISSWWW do a January sale!!!
 
Thanks Monkeh,

I guess I am after a true RMS type of meter or an analogue one then. That fluke you mentioned looks pretty decent. Might wait and see if ISSWWW do a January sale!!!

True RMS has nothing to do with so called 'ghost voltages'. They arise due to capacitance on the cable, and the normal input impedance of a digital meter (10M nominally) is insufficient to dissipate the voltage. Meters like the 117 have a specific mode with a low input impedance (3k) which will quickly dispose of such voltages.

It'll also get a tad warm if you hook it up to a live conductor for more than a few seconds..
 
Thanks Monkeh,

I guess I am after a true RMS type of meter or an analogue one then. That fluke you mentioned looks pretty decent. Might wait and see if ISSWWW do a January sale!!!

True RMS has nothing to do with so called 'ghost voltages'. They arise due to capacitance on the cable, and the normal input impedance of a digital meter (10M nominally) is insufficient to dissipate the voltage. Meters like the 117 have a specific mode with a low input impedance (3k) which will quickly dispose of such voltages.

It'll also get a tad warm if you hook it up to a live conductor for more than a few seconds..


Oh, OK.

I googled the fluke and then googled true RMS. I found a sales vid on youtube that stated that many meters chop up the AC sinewave to get an average, whereas a true RMS type meter does not. It also mentioned that the voltage measured can be significantly different between the two types of meter.

I shall bow to your wisdom and do a bit of research tomoro to understand what you are saying re input impedances.

Thanks again.
 
I googled the fluke and then googled true RMS. I found a sales vid on youtube that stated that many meters chop up the AC sinewave to get an average, whereas a true RMS type meter does not. It also mentioned that the voltage measured can be significantly different between the two types of meter.

Yeah, but on a mostly unbutchered sine wave it doesn't make much difference.

I shall bow to your wisdom and do a bit of research tomoro to understand what you are saying re input impedances.

Think of a multimeter in voltage mode as a very high impedance short across the connection points. If you lower the impedance, more current can flow, which will discharge the capacitance of the wire.
 
110V on a floating CPC seems to be about the average reading I come across when measured with a high impedence meter.

Why did you have a new circuit energised with the CPC disconnected?
 
110V on a floating CPC seems to be about the average reading I come across when measured with a high impedence meter.

Why did you have a new circuit energised with the CPC disconnected?

Was thinking just the same thing! Why are you trying to make the MFT do something it is not designed to do?
 
110V on a floating CPC seems to be about the average reading I come across when measured with a high impedence meter.

Why did you have a new circuit energised with the CPC disconnected?

I was waiting for that to come up!

Empty property, dark outside, lights on. Forgot to do R2 as a dead test due to a distraction earlier in the day. I did the R2 and felt the merest of tingles, that's why I measured between the floating cpc and the rod.
 
I thought R2 is a dead test? (As is also an R1+R2), you shouldn't be getting a tingle
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top