Dodgy wiring

hightower · Mar 14, 2016

Been speaking to a neighbour who had an outside light installed a couple years ago. Apparently the light worked for a while.

The 'electrician' apparently wired the neutral for the fcu to the switched line of the ceiling rose. I'm guessing the line of the fcu was wired in to the permanent line of the same ceiling rose, but my neighbour didn't gave that info, only about the neutral.

I haven't had chance to draw this out and I'm having trouble visualising it, but I can't see how it worked for a while.

Think I'm gonna need to draw it out. ..

Dave OCD · Mar 14, 2016

With the light it was fed from switched off there may have been a return path of sorts back through other lamps in the circuit -possibly.

Pete999 · Mar 14, 2016

You need to have a look mate, never trust heresay

hightower · Mar 14, 2016

Pete999 said:
You need to have a look mate, never trust heresay

It was rectified a few weeks after it was done. It's a new build so BETT came to have a look 2 years ago, and that's the note they left her, that the neutral was wired to the sw line.

Doesn't mention the rest of the wiring, but really struggling to see how it would work at all.

Apparently this error has voided her warranty on her new build though.

PEG · Mar 14, 2016

Who is the warranty with? Coz i have pals whose new-builds included wrong sewer connections,roofs with MC Ischer gutters,non-functioning drains,bio-filters,and much,much more...and they never incurred a voided warranty...

happysteve · Mar 14, 2016

Second rule of Steve: "If in doubt, draw it out."

Pretty sure you're smart enough to work it out from that, assuming you can read my scrawls!

I've made assumptions about the rating of the internal (100W) and external (10W) lights. Essentially, the voltage across the external light is related to the ratio of the consumption of the two lights. If they were both 10W, then there would be 115V across the external light.

In the scenario above, 209V is plenty to light up the exterior light, whereas 21V probably won't even make the internal light glow.

All appears to work, until the customer turns on their internal light. At that point, the exterior light turns off.

Hope that's useful, anyway!

GETWIRED · Mar 14, 2016

Suppose that spark was having a bad day lol

going to open myself to flak here but applying the theory to that fault is pretty brilliant..

hightower · Mar 15, 2016

Thanks steve, you always seem to come up with the goods

hightower · Mar 15, 2016

PEG said:
Who is the warranty with? Coz i have pals whose new-builds included wrong sewer connections,roofs with MC Ischer gutters,non-functioning drains,bio-filters,and much,much more...and they never incurred a voided warranty...

It's a BETT home but the additional light was done by a local electrician. My neighbour caked bett out when things started going wrong and they said the warranty was no more because of the incorrect wiring

hightower · Mar 15, 2016

happysteve said:
Second rule of Steve: "If in doubt, draw it out."

What's the first rule of steve?

happysteve · Mar 15, 2016

hightower said:
What's the first rule of steve?

First rule of Steve: don't talk about the Rules of Steve.

hightower · Mar 15, 2016

I've just redone the maths because the outside light was this model: 120W Anti-Light Pollution PIR Floodlight - Black

Outside Light:
P = 120w
V = 104.4v

Inside Light:
P = 100w
V = 125.6v

Would those voltages be enough to power each to some degree?

telectrix · Mar 15, 2016

if the N of the outside light is connected to a switched L, then if the switched L is not energised, it's seen as a N by the outside light as it has no potential. this is how location indicators on switches work. they'e ON when the switch is OFF and ON when the switch is ON.

hightower · Mar 15, 2016

telectrix said:
if the N of the outside light is connected to a switched L, then if the switched L is not energised, it's seen as a N by the outside light as it has no potential. this is how location indicators on switches work. they'e ON when the switch is OFF and ON when the switch is ON.

Yeah but in this case the outside light is in series with the inside light, and the voltage applied to each light isn't the full 230v. As Steve said, if both the fittings were near the same then they would equally share the voltage. When the switch is turned on the outside outright wouldn't work, but what would happen when the switch is off if both lights had 115v each? Would it be enough to power both but dimly? If so, how didn't my neighbour notice the bedroom light was constantly on?

Or would it mean the bedroom light would only come on when the PIR triggered? She can see the outside light works, but doesn't realise there's a disco happening in her bedroom since she's outside triggering the PIR.

telectrix · Mar 15, 2016

could well be that. the voltage shared between the 2 lights in series. a lot of LED drivers have a large voltage tolerance, some around 90V to 240V

happysteve · Mar 15, 2016

hightower said:
Or would it mean the bedroom light would only come on when the PIR triggered? She can see the outside light works, but doesn't realise there's a disco happening in her bedroom since she's outside triggering the PIR.

Bingo!

As Telectix says, some are more tolerant of voltage than others. If they're both tungsten/tungsten-halogen, then their brightness will depend on the voltage. If they're LEDs, then it won't (much) - they either work or they don't.

An interesting thing about incandescents: their resistance when off is a lot lower than when they're turned on, because as wires get hotter their resistance goes up. So a 100W GLS lamp will only have a resistance of about 530 ohms when it's got about 230V across it - below that, the resistance will be (I reckon, anyway) less.

happysteve · Mar 15, 2016

hightower said:
What's the first rule of steve?

The first rule of Steve is actually:

Write down what you know, in terms you understand.

E.g.:

Question, as written on the exam: "A load has power factor correction applied, so that the resulting impedance is 10 ohms when pf=1. The potential difference across the load is 230V a.c. What is the current through the load?"

What you write: R=10 (ohms), V=230 (volts); I = V/R

That way, you don't get muddled up by the way they phrase the question - they go on special courses on how to set questions this way... I think it's called the "sneaky b@5tard course"...

If that doesn't make things crystal clear, then you go to the second rule of Steve: If in doubt, draw it out.

hightower · Mar 15, 2016

happysteve said:
The first rule of Steve is actually:

Write down what you know, in terms you understand.

E.g.:

Question, as written on the exam: "A load has power factor correction applied, so that the resulting impedance is 10 ohms when pf=1. The potential difference across the load is 230V a.c. What is the current through the load?"

What you write: R=10 (ohms), V=230 (volts); I = V/R

That way, you don't get muddled up by the way they phrase the question - they go on special courses on how to set questions this way... I think it's called the "sneaky b@5tard course"...

If that doesn't make things crystal clear, then you go to the second rule of Steve: If in doubt, draw it out.

I've even started doing that in class now - the lecturer might say "take this down. A voltage of 200v is applied across a resistance of 10ohms. Calculate the current."

Rather than write it all down my notes look like this:

V = 200V
R = 10Ohms
I = ?

Dave OCD · Mar 15, 2016

Old Dave got this right in post 2, but Happysteve and others explained it far more eloquently with the maths to back it up. :bulb2::laugh3:

hightower · Mar 15, 2016

Dave OCD said:
Old Dave got this right in post 2, but Happysteve and others explained it far more eloquently with the maths to back it up. :bulb2::laugh3:

You did indeed dave, thanks has been sent your way in the shape of fake Internet points [emoji38]