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Hello all,

I would appreciate some advice on safely working with wires downstream of a fused connection unit (FCU).

Some background: An electrician added some lighting in my loft. It appears that he tapped off of the smoke alarm circuit (since the lights go out if I isolate the breaker in the consumer unit). I traced the wire back to a hole in a wooden board at the level of the loft floor. It goes into the switch that operates the 2-3 lamps he installed.

The first thing that struck me as odd was despite killing the lights in the loft by switching off the smoke alarm breaker in the consumer unit, when I checked the wires with a contact meter and a volt pen, they both indicated that the wires were live. The only way to get no live wires detected was to kill the power at the main circuit breaker. Why?

I've attached a sketch of what it is that I'm trying to acheive. On the left side is the existing cable that went into the light switch. On the far right side is the wiring that goes to the light switch. Everything between the two and above and below it are new items.

The plan goes as follows:

1. Feed the smoke alarm supply cable into FCU, then into a junction box
2. The 3 o'clock wire goes to another FCU, then to the light switch
3. The 12 and 6 o'clock wires go into another FCU, then to a double or triple socket

I'm going to be using the double socket to power a security camera (with one spare socket for future-proofing), and the triple socket will power two security cameras (with one spare). The AC/DC converters of the cameras are rated at 0.3 A max. So I'm not concerned about overloading the circuit.

The reason for all the FCUs are to allow me to isolate various arms if needed to make changes or to progress with work (one hour here and there - I don't have a solid block of time to finish it in one go). Also I suppose they provide some added protection even though it's overkill as the loads will be very low.

So far I've managed to do steps 1 and 2. I wired everything correctly and the lights switch on and off as expected either from the light switch, or the FCU nearest them, or the FCU nearest the supply.

The problem is that despite having isolated the supply at the first FCU, my contact meter and volt pen detect live wires. I also pulled out the fuse in the first FCU but I still detect live wires downstream. I can't understand why but I'm guessing it's related to the previous inability to isolate the wiring from the smoke alarm's circuit breaker.

Why is the smoke alarm circuit breaker or FCUs not isolating in the way that I'm expecting them to?

Best regards,
Cris
 
TL;DR
To expand on my final question, also why did pulling out the fuse not achieve isolation?

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A volt pen is a wholly unreliable way to check if something is dead, it may indicate it is live when in fact it is not. What is a contact meter.
 
Hi Westward10, you're absolutely right. By contact meter I meant wire/stud detector. In fact I already knew that my wire detector is unreliable (I guess it can't distinguish between live/dead cable) so it was really secondary to my volt pen, which I thought could be relied on.

But I noticed this evening that if I probe some cable not connected to anything it beeps and lights up red. Or maybe two beeps doesn't mean live, but only when it continues to beep...

So ultimately I used a multimeter to confirm that the cables were dead after switching off the supply into the first SCU.

So if volt pena arw unreliable why does anyone use them?

I suppose that a proper functioning voltmeter (with infinite resistance) should be OK to use to test live cables but I'm a bit twitchy about probing potentially live wire.

If not a voltmeter what doe others use?
 
Hi Westward10, you're absolutely right. By contact meter I meant wire/stud detector. In fact I already knew that my wire detector is unreliable (I guess it can't distinguish between live/dead cable) so it was really secondary to my volt pen, which I thought could be relied on.

But I noticed this evening that if I probe some cable not connected to anything it beeps and lights up red. Or maybe two beeps doesn't mean live, but only when it continues to beep...

So ultimately I used a multimeter to confirm that the cables were dead after switching off the supply into the first SCU.

So if volt pena arw unreliable why does anyone use them?

I suppose that a proper functioning voltmeter (with infinite resistance) should be OK to use to test live cables but I'm a bit twitchy about probing potentially live wire.

If not a voltmeter what doe others use?

Volt sticks can be useful in certain circumstances, but it is essential to know how they operate.

As mentioned above, they are certainly not suitable to prove a circuit is dead and safe to work on.
 
So if volt pena arw unreliable why does anyone use them?

If not a voltmeter what doe others use?

People use volt pens to test for live indication where terminals are not accessible. I personally don't use one at all. They are not to be used for safe isolation.

Electricians generally used approved voltage indicators compliant with GS38 for safe isolation. These should be used with correct safe isolation procedure.

 
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