Insulation Resistance query

[Munkey]

Could it not be down to harmonics? All that electronic stuff plays havoc and lots of distortion on the mains supply. This coupled with an RCD could definitely cause some nuisance tripping. Is that not why they have functional earths for installations with lots of electronics and Residual Current Monitoring devices in some cases? Or am I talking balls?

 

[telectrix]

Doesn't a sky box send out 50v to the dish?

13.6V to the dish

 

[Guitarist]

Could it not be down to harmonics? All that electronic stuff plays havoc and lots of distortion on the mains supply. This coupled with an RCD could definitely cause some nuisance tripping. Is that not why they have functional earths for installations with lots of electronics and Residual Current Monitoring devices in some cases? Or am I talking balls?

I've had lots of sky boxes over the years coupled with 30ma RCD's and never had one nuisance trip. Could be a faulty unit tho, hence wondered if a different box had been tried.

 

[ackbarthestar]

If you suspect that 'Rupert's magic box' was the likely cause of the disturbance, then I would have thought you just pull the plug on it (that would be disconnect it in its entirety ! ) before testing !
Its not as though you require a parliamentary debate is it?

 

[Amp David]

Its not as though you require a parliamentary debate is it?

Sorry boss, get out the wrong side of the bed or something.

Its a forum isn't it?

Goon:skep:

 

[mhar]

Doesn't a sky box send out 50v to the dish?

13 / 17v switchable depending on polarity.
Isn't a sky box always on even when in standby? Haven't got one myself to test but thought that it was always on for epg / software / channel parameter updates and for sky to address the box to check for phone line connections. Always thought that it was only the picture producing circuitry that shut down.
The outer braid of the coax is grounded to the chassis

 

[ackbarthestar]

Sorry boss, get out the wrong side of the bed or something.

Its a forum isn't it?

Goon:skep:

Lol, No offense taken Old Boy :angel_smile:

How can you seriously debate that you have a low IR of 10.x M Ohms and at the same time cogitate about 'Rupert's box' that may or may not be the culprit which is still connected into the circuit.

Surely the easiest solution would be to disconnect it completely. Then when you find that the result is 10.x M Ohms you can then move onto a damp socket or dirt trapped in some socket terminals.

10.x M Ohms is not really a problem for an old circuit. Try testing it at 1000V for a minute and see whether or not the IR value begins to decrease....

 

[Amp David]

Main question was how can something that is double insulated give low readings between live conductors and CPC?

Question answered.

The reading I obtained is fine, we all know that, I have never questioned it.

 

[WDMDL]

Here's one you should know about.... the primary side of MOST switch-mode power supplies on DOUBLE-INSULATED equipment like Sky boxes, TVs etc.....

The supply enters the box, passes through some filters and then is rectified with a bridge-rectifier and smoothing capacitor arrangement. It hasn't reached a transformer at this stage.
The negative side of this (300-340V DC) primary supply is connected to chassis via a network of components, usually a high voltage rated 4.7 Meg resistor (or sometimes 2 in series), paralleled by a small-value (say 4.7nF) high voltage or Class Y rated capacitor.
In the service manual, these are designated safety-critical components that must only be replaced by the exact type specified.

You can see this network in the diagram extract below (it's for a TV)
The pitchfork ground symbol is the primary DC negative, the flat ground symbol is chassis.
The exclamation triangle denotes the safety critical components



That's why you get a tingle when leaning on the sky box, VCR etc and trying to connect the aerial with the other hand!
It's also why they insist that ALL system components must be unplugged from the mains before interconnecting.

That's where your 10 and a bit Megohms is coming from!

Simon.