What could cause my voltage to drop to 28V??

[HappyHippyDad]

Evening all..

I have just tested a cable that I had put in place myself for a smoke detector and I am only getting 28V L-N. I am getting 240V L-E.

When I first put it in it tested 240V. Since then the builder has been in and put the ceiling up, I'm not saying this is the cause but this is the only thing that has changed since my first test.

The cable was taken from a MF JB that is now hidden above the ceiling. I have tested a light that is from the same JB and it all tests at 240V.

If some how the neutral has been damaged how does this cause the voltage to drop? There would have to be a pretty big jump in resistance for it to drop that much and even if the faulty neutral was just brushing the non-faulty neutral it would still read 240V?

 

[amlu]

continuity/insulation resistance test on the circuit?

 

[Darkwood]

High impedance faults are common, a broken N or loose terminal can cause arcing which can leave carbon tracks ..it may be these that you are getting a reading from it may also be the electronics in the SD giving a feed back while the N is missing altogether... did you drop the cable off the SD when you tested it?

 

[Marvo]

Evening all..

I have just tested a cable that I had put in place myself for a smoke detector and I am only getting 28V L-N.

28 volts sounds like an arbitrary coupled voltage that you tester is picking up. Isolate the circuit and check for end to end continuity of the L, N + E. Also IR test, 28v L-N could indicate that the neutral is live. It's almost a certainty the builder has damaged the cable when boarding the ceiling. Did you buy that inspection camera you were thinking about in your other thread. You might get to use it by the looks of things, make some 10mm holes in the boards and give it a whirl .

 

[HappyHippyDad]

continuity/insulation resistance test on the circuit?

It was 7pm and the customer wanted a bit of the evening to themselves.

High impedance faults are common, a broken N or loose terminal can cause arcing which can leave carbon tracks ..it may be these that you are getting a reading from it may also be the electronics in the SD giving a feed back while the N is missing altogether... did you drop the cable off the SD when you tested it?

There had not been any load on the cable Darkwood so it may not be your first idea. The cable was disconnected from the smoke detector when I tested.

 

[PEG]

Just for interest,what did you measure your 28v with? cheers

 

[HappyHippyDad]

28 volts sounds like an arbitrary coupled voltage that you tester is picking up. Isolate the circuit and check for end to end continuity of the L, N + E. Also IR test, 28v L-N could indicate that the neutral is live. It's almost a certainty the builder has damaged the cable when boarding the ceiling. Did you buy that inspection camera you were thinking about in your other thread. You might get to use it by the looks of things, make some 10mm holes in the boards and give it a whirl .[/QUOTE]

I shall do all of those tests when I return..

Yes.. I did buy the camera Marvo, and I didn't give a thought to using it!!! Now I'm quite excited at the prospect! Thanks for reminding me :smile5:

 

[HappyHippyDad]

Just for interest,what did you measure your 28v with? cheers

Fluke T5.

 

[PEG]

Fluke T5.

Did it put the LEDs' on,or just the digital? ...only for info:yes:

 

[amlu]

have a look on continuity both N and L. may be either poor termination somewhere (wago not pushed in properly?) or builder put a screw in the cable (insulation resistance between live/neutral and wall or ceiling may help to find out).

 

[HappyHippyDad]

Did it put the LEDs' on,or just the digital? ...only for info:yes:

Cant remember Peg.. sorry. Just remember seeing the 28V staring at me and thinking.. oh dear!

 

[PEG]

Cant remember Peg.. sorry. Just remember seeing the 28V staring at me and thinking.. oh dear!

No probs...it was just to get a feel for how that particular tester (T5),does its' thing...

I have set up deliberate faults in the past,and then shown how many different results can be attained,by using a variety of test instruments.

...You can see a lads confidence in a specific instrument,trickle away,when he sees differing results,from other "respected" machines.:yes:

 

[Paul.M]

Go to the part of the building the plasterer worked, do end to end continuity tests on all conductors in between each smoke detector in that section (including the signal wire if this is 3 core hard wired. Dont forget about the interlinked conductor!).

Once you have found the section of cable that has a screw through it, take your hammer out, make a hole and then phone the plasterer with the good news. Job done

 

[pennychew]

Fluke T5.

I was working in a factory the other day putting a stainless shelf up for a microwave and it felt like i was getting a little tingle in my arm hair from the side of the microwave when i touched that and the galv conduit drop beside it. I had my fluke T5 in my bag so i checked between the two and was getting nearly 105v.

Went and got my fluke 117 multimeter and put it on the low impedance setting and it was only actually reading 0.4v. I only really trust my T5 for checking something is dead or continuity tests now

 

[Richard Burns]

If you measure the Neutral to earth voltage this should give you an idea of the state of the neutral; 230V if disconnected.

 

[Dave OCD]

Was it not possible to get enough clearance between the underside of the joists and the cables to avoid a screw reaching the wiring - assuming that turns out to be the problem ?

 

[Marvo]

Soooo.....did you find the screwhole....ahem I mean problem?