TV Power Cable Extension

Quite - 'earth continuity' betyween what and what? If it has a floating output and (like every one I've seen) no external conductive parts, the concept of 'earth continuity' seems irrelevant/meaningless.
Earth continuity between the earth pin of the plug and whatever parts of the system rely on the earth as part of their protection strategy.

For a reductio-ad-absurdium argument consider what would happen if I took a Class 1 metal cased appliance and wrapped it in clingfilm. The clingfilm is an insulating material but it's clearly in no way an adequate insulation barrier for electrical safety. So the earthing of the metal case still forms part of the protection strategy.

A similar argument applies to power supply design. Capacitors between mains earth and the input only need to be rated to basic insulation standards. Capacitors between mains earth and the output don't need to be safety rated at all.
 
Sponsored Links
Earth continuity between the earth pin of the plug and whatever parts of the system rely on the earth as part of their protection strategy.
Yes - but, as I implied, if there are no "parts of the system [which] rely on the earth as part of their protection strategy" (which appears to be the case with my laptop charger) then there would seem to be no relevant 'continuity' to test, would there?
For a reductio-ad-absurdium argument consider what would happen if I took a Class 1 metal cased appliance and wrapped it in clingfilm. The clingfilm is an insulating material but it's clearly in no way an adequate insulation barrier for electrical safety. So the earthing of the metal case still forms part of the protection strategy.
Sure, but the casing of my laptop charger appears to be made of very 'solid' plastic - seemingly at least as 'solid' as the "reinforced insulation" of many a Class II item.

Kind Regards, John
 
I am not sure where this is going but my last TV, from 2009 I believe, a Samsung LE40B550 had an earth but no exposed metal.
I'm not sure where it is going, either, but I would again ask.... if, although it "has an earth" it has no exposed metal, what relevant thing(s) is/are actually being 'earthed' by 'the earth'?

Kind Regards, John
 
Fair enough - that's the answer I was asking for. Never having owned (and rarely handled) a MacBook, I didn't know that they had a metal body.



Kind Regards, John
yes, the MacBook has a metal body. But the supply is from a 5 volt DC power supply. Via ( these days) a USB-C connector.
Earthing of the case is not a consideration.
 
Sponsored Links
I'm not sure where it is going, either, but I would again ask.... if, although it "has an earth" it has no exposed metal, what relevant thing(s) is/are actually being 'earthed' by 'the earth'?
Lets say I'm designing an isolated SMPSU with a floating output. Lets further say that the output will be connected either directly or via non safety rated components to touchable metalwork. This is nearly always the case with AV and IT equipment. Even if the equipment itself has plastic boxes, there are plenty of connectors with exposed metal used on AV and IT equipment.

SMPSUs are noisy, to meet EMC standards I need to create a high frequency path between the input and output sides of the transformer. Otherwise the interaction of the parasitics in the transformer with the switching action of the converter will result in unacceptable RF emissions.

I could just put a capacitor between input and output sides of the circuit, many smaller SMPSUs do, but if I do that then.

1. The capacitor has to be rated to safety standards equivilent to "double or reinforced insulation". IIRC this means a "Y2" capacitor.
2. The capacitor value has to be small enough that the touch current is acceptable. Multiple items of equipment may be interconnected, so the total touch current may be several times the touch current from any one power supply.

Alternatively I could take two capacitors. Connect one between the input and mains earth, and the other between the output and mains earth. Now my capacitor selection becomes much easier.

1. The capacitor between the input and mains earth only needs to provide protection equivilent to "basic insulation". IIRC this means a "Y1" capacitor.
2. The capacitor between the output and mains earth doesn't need to be safety rated at all.
3. The capacitor value can be larger, because currents in the capacitor will give rise to earth leakage currents rather than touch currents.

The "relavent thing being earthed" is then the point where the input-earth capacitor and the output-earth capacitor meet. In an ideal world one would test earth continuity from the earth pin on the plug to that point.

In the real world of course that isn't possible, so one just has to do the best that is reasonablly practicable. For example for a power supply with a detatchable lead testing earth continuity to the end of said detachable lead.
 
Last edited:
You may be surprised ;)
For example, every Unibody MacBook (using the official MagSafe2 charger), is earthed through the PSU.

Fair play. Every laptop that I have owned, regardless of the laptop shell has had round port connectors That said, yes the supply to the power brick may indeed have an earth.
 

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