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Safety Earth Question

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Safety Earth Question

Postby jessemcgoldrick » Sat Apr 24, 2021 12:32 pm

I have two pieces of equipment; a media player and an amp. Neither is double insulated, therefore both use a 3 pin earthed power cable and I assume both have an internal connection between their metal chassis and the mains earth connection. My questions are:

1 - If mains earth became disconnected from one device, would an audio cable connecting both devices provide a fallback safety earth route through the mains earth connection of the other device? (To be completely clear I’m not suggesting or considering that a mains earth connection should be disconnected from any device intentionally, I’m just wanting to understand in theory whether the audio cable connection between two earthed devices can serve as a fallback safety earth connection or whether audio ground connections are unable to serve this function). I have seen cables with an audio jack at one end and a mains plug at the other which allow floating ground devices to be earthed so I assume this means that audio cable ground connections are typically connected to mains earth in a mains earthed device?

2 - Does the type of cable make a difference to the above? For example if an unbalanced RCA cable was used or a balanced XLR?

3 - If an audio cable DOES provide a route to mains earth through a connected device, is it possible that in the event of a malfunction electricity would ever use this route instead, or in addition to, it’s own earth connection? I assume that when a device has it’s own mains earth connection this route would be shorter (or have less resistance) and therefore teh alternative route through a connected device would remain safe?

4 - The amp has ground terminal connections available for phono connection of a turntable. Would this ground terminal likely also provide a route to mains earth through the amp power connection?

5 - Is there a typical method for testing whether the metal chassis of a device is successfully connected to mains earth via audio cable to a mains earthed device?

Many thanks for any explanations or info. Again, I’m not suggesting or considering that mains earth should ever be intentionally disconnected from a device, I’m just wanting to understand the routes that can provide a safety earth connection.
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Re: Safety Earth Question

Postby Folderol » Sat Apr 24, 2021 1:38 pm

All of them could give some assistance, but none would provide reliable protection as they could be disconnected at any time for perfectly valid reasons.
In fact, there is one circumstance where they could turn a painful shock into a fatal one.

This is where one has become live while the other is earthed but they are not yet connected. As you plug in you will have one hand holding the metal shelled plug of one while the other hand is resting on the chassis of the other kit to keep it steady. At that point there is a direct path through your heart.
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Re: Safety Earth Question

Postby Hugh Robjohns » Sat Apr 24, 2021 1:55 pm

jessemcgoldrick wrote:1 - If mains earth became disconnected from one device, would an audio cable connecting both devices provide a fallback safety earth route through the mains earth connection of the other device?

Kind of... But that arrangement would be inherently UNSAFE and potentially LETHAL, particularly when audio connections were being made or disconnected.

I’m just wanting to understand in theory whether the audio cable connection between two earthed devices can serve as a fallback safety earth connection or whether audio ground connections are unable to serve this function).

No, an audio cable cannot ever serve as a fallback safety earth connection. Never. Nope. Nyet.

The screen of an audio cable is not rated to carry significant current, so in the event of a major fault the audio cable would quickly (instantly?) burn out and fail in its provision of a 'safety earth'.

Also... there's no guarantee that the audio ground will be internally connected sufficiently robustly to the chassis earth of the equipment anyway.

I have seen cables with an audio jack at one end and a mains plug at the other which allow floating ground devices to be earthed so I assume this means that audio cable ground connections are typically connected to mains earth in a mains earthed device?

Kind of... It is fairly normal to link the audio ground to the chassis ground at one point (the star-point) inside a class-1 device such as a hi-fi power amp, powered speaker, mixer, etc.

But this is not designed or intended to act as a safety earth. It's simply to ensure effective electrostatic screening, which is a very different thing. Moreover, some products intentionally make this connection via a small resistor to limit the possibility of audible ground-loop currents (sometimes the resistor can be switched in/out via a 'ground-lift' switch), and some don't connect the two at all...

The custom cable you describe is used purely for ensuring effective electrostatic screening, not for mains safety protection.

Most audio installations rely on having at least one class-1 device in the system (usually a power-amp or mixer, for example) that is grounded via its mains connection. That earth connection can then be shared out to other connected class-2 (double-insulated) devices via the audio cable screens to extend electrostatic screening reasonably effectively.

That said, it's becoming increasingly common to find installations where all the audio devices are class-2 double-insulated and there is no mains earth connection at all. Sometimes these 'floating' installations work perfectly well... but where they don't (usually where an electric guitar is involved!) it is often beneficial to provide an external earth for screening purposes via the type of special custom cable you describe -- essentially just replicating the normal grounded screen provision of a power amp (etc).

2 - Does the type of cable make a difference to the above? For example if an unbalanced RCA cable was used or a balanced XLR?

No. Neither provide a safe alternative to a proper mains earth connection.

3 - If an audio cable DOES provide a route to mains earth through a connected device, is it possible that in the event of a malfunction electricity would ever use this route instead, or in addition to, it’s own earth connection?

Current follows the route of least resistance... so if one device lost its mains earth any resulting fault currents would flow briefly through the audio cable screens to a properly grounded device until that cable either fused or the mains supply tripped out.

Meanwhile, anyone plugging or unplugging the equipment could be at risk of lethal electric shocks...

4 - The amp has ground terminal connections available for phono connection of a turntable. Would this ground terminal likely also provide a route to mains earth through the amp power connection?

Yes. The turntable ground terminal is normally connected to the amp's chassis and thus via the mains cable to the mains earth. But again, it's intended for electrostatic screening purposes, not a safety ground connection under fault conditions.

5 - Is there a typical method for testing whether the metal chassis of a device is successfully connected to mains earth via audio cable to a mains earthed device?

Yes. Use a digital multi-meter to measure the resistance between the chassis and the mains plug earth pin. Better still, use a dedicated PAT tester which will check the impedance and current capability are sufficiently good to qualify as a safety earth.

Again, I’m not suggesting or considering that mains earth should ever be intentionally disconnected from a device, I’m just wanting to understand the routes that can provide a safety earth connection.

Class-1 mains-powered devices are equipped with a safety earth connection via the three-core mains cable and this MUST always be maintained. No other configuration is ever acceptable.

Questions like yours are inherently disturbing... perhaps you can help put my mind at rest by explaining your interest? Are you trying to resolve a ground-loop problem?
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Re: Safety Earth Question

Postby jessemcgoldrick » Sun Apr 25, 2021 1:11 am

Hugh Robjohns wrote:Questions like yours are inherently disturbing... perhaps you can help put my mind at rest by explaining your interest? Are you trying to resolve a ground-loop problem?

Ha, understandable, yes I’ll give some background. I recently asked for help on these forums regarding a grounding connection issue which you were able to solve for me (thanks again). This led to me spending more time trying to read about and understand grounding better in general as it’s always been a bit of a mystery to me. In the process I came across several articles/diagrams which showed two devices connected via an audio cable with what appears to be a direct connection in both devices from audio ground to chassis and chassis to mains earth.

An example is Figure 7 on page 4 of this interesting article: http://tech.juaneda.com/en/articles/Ground_and_safety_earth.pdf
(Apologies, I couldn’t seem to link to only the diagram itself)

I live in a really, really old building with almost prehistoric electrical wiring and fittings (for example we once had live current coming out of the screw on a socket housing as the cable insulation inside was so old and brittle it had crumbled away) and seeing diagrams like the above it occurred to me that if for any reason there was ever a problem with the earth connection in one of my wall sockets or indeed in the cable or plug, perhaps I would still be protected to some extent, at least temporarily, by another device connected via audio cable and safely earthed via another socket...

My questions are just theoretical and I’m not secretly trying to find an alternative to existing safety earth connections, I’m just interested in understanding grounding and safety earth and the various routes that electricity takes through my system. I chose to use the example of a specific connection between two devices but there isn’t any real world problem that I’m trying to solve after you already helped me solve the only issue I had.

Hopefully the above context will put your mind at rest. I really appreciate the time you’ve taken to provide such detailed and informative answers (not just to this post, I have learned a great deal by reading contributions from you and others throughout the SOS forum).

As I now understand it, audio cables may technically provide a route to earth and could possibly trip out the mains supply in the event that a device’s own earth connection is lost but they cannot in any way whatsoever be considered a ‘fallback’ safety connection and do not reduce the risk of a fatal electric shock.
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Re: Safety Earth Question

Postby James Perrett » Sun Apr 25, 2021 1:35 am

jessemcgoldrick wrote:I live in a really, really old building with almost prehistoric electrical wiring and fittings (for example we once had live current coming out of the screw on a socket housing as the cable insulation inside was so old and brittle it had crumbled away)

If you own this property then get it rewired IMMEDIATELY.

If you rent then the landlord needs to fix the wiring.

This is exactly the sort of thing that could cause a fire or electrocute you. Don't wait - do something about it NOW before something really bad happens!
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Re: Safety Earth Question

Postby Folderol » Sun Apr 25, 2021 9:34 am

That is indeed an extremely dangerous situation. For PVC to go so brittle it crumbles, it has to have been extremely hot for a prolonged period. The other possibility, VIR, is even more dangerous. This is the ancient rubber insulation that stopped being fitted by the late 1950s, and is known to crumble on exposure to air.
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Re: Safety Earth Question

Postby Hugh Robjohns » Sun Apr 25, 2021 1:57 pm

I concur. The situation you describe would not pass an electrical inspection (which is now mandatory in rented property at least every five years).

I'd get an electrician in pronto to check the place out...
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Re: Safety Earth Question

Postby Hugh Robjohns » Mon Apr 26, 2021 12:05 pm

jessemcgoldrick wrote:In the process I came across several articles/diagrams which showed two devices connected via an audio cable with what appears to be a direct connection in both devices from audio ground to chassis and chassis to mains earth.

An example is Figure 7 on page 4 of this interesting article: http://tech.juaneda.com/en/articles/Ground_and_safety_earth.pdf

I've just had a chance to read through that article, and I think there are several misunderstandings and slightly dodgy advice in there.

For example, his 'one step further' idea of adding an extra earthing point is really not a good idea, and actually only replicates what a properly designed product should do anyway. Ideally, the power supply reference ground (post regulators) and the audio circuitry ground should meet at what is known as the 'star point' which also connects to the chassis and incoming mains safety earth.

His comments about ground-loops appears to take into account the voltage differences between different mains socket earths due to ground currents flowing in the building's mains wiring. His arrangement of connecting each device to earth doesn't cancel parasitic currents in the audio cables at all!

It is true that flipping a fig-8 mains connector around on a Class-2 double insulated device can alter the parasitic currents quite significantly.

The negative effect of parasitic currents flowing across the audio cable screen is heavily dependent on the way the product is designed. The downside of unbalanced audio connections is that the cable screen also serves as the audio signal reference voltage, and so any currents flowing in the screen will affect the reference voltage and thus become embedded within the audio signal.

And that's why balanced audio connections are so much better. The cable screen plays no role in the transferring the audio signal voltage, so parasitic currents flowing in the cable screen don't affect the audio reference.

...Assuming the product is designed properly and the cable screen is taken directly to the chassis and not the audio circuitry ground reference (which is sadly very common, especially in legacy gear)!
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