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Ground loop hum fix - is this safe?

T

TimR

pfm Member
Hi all,
I'm after some basic safety advice in case I've just done something horribly dangerous...

It all started with a ground loop hum which came about after I disconnected my PS3 from the mains. This is one of 4 HDMI devices connected to my AV amp, but apparently the only one with a properly earthed 3-pin power cable.

I figured that removing this earth connection might be part of the problem, so as an experiment, I ran an earth wire from a 3-pin plug to a screw on the case of my Virgin TiVo box. The problem immediately diminished, so I continued to daisy chain this wire to my DVD recorder and TV, and the hum has gone.

So can I leave it like this?
 
Sure you can?
A properly designed electronic component is earthed via the 3 way plug as you discovered. Most of the time, this earthing is taken from your mains cable and internally immediately rooted to the metal casing of the component. Which is the part you would touch during normal operation.

So connecting the earthing of all components together and rooting them to ground is not a problem at all. Just make sure the connection at the plug point is solid and won't accidentally short over to the live phases.
Now that would really be dangerous.
 
Exhibit "A":

A Naim 250 Amp. See the green/yellow earth wire top right? Attached straight to the casing.

nap250inners.jpg
 
Star grounding. Always works in the most horrific area for ground loops - car audio. Earth everything back to the same place and the loop can no longer exist...

I can't see a problem with what you are doing, but it would be better if you grounded each component directly using its own ground. Also check that your extension blocks (if you are using any) have continuity on the ground side..
 
Rug Doc said:
Star grounding. Always works in the most horrific area for ground loops - car audio. Earth everything back to the same place and the loop can no longer exist...

I can't see a problem with what you are doing, but it would be better if you grounded each component directly using its own ground. Also check that your extension blocks (if you are using any) have continuity on the ground side..
Click to expand...

Absolutely agreed, though not always possible or simple to achieve. Earth loops are not always simple, or easy to figure out, in which case it comes down to, hopefully, disconnecting ONE earth connection so that you break the loop - by definition whatever you disconnect from earth must be earthed via something else, otherwise there would not be a loop.

Topa - that is a legal requirement.
 
Indeed, and it's (in car audio) usually the RCA ground carrying the earth, instead of the proper ground cable ... A very dangerous situation to be in..
 
TimR said:
Hi all,
I'm after some basic safety advice in case I've just done something horribly dangerous...

It all started with a ground loop hum which came about after I disconnected my PS3 from the mains. This is one of 4 HDMI devices connected to my AV amp, but apparently the only one with a properly earthed 3-pin power cable.

I figured that removing this earth connection might be part of the problem, so as an experiment, I ran an earth wire from a 3-pin plug to a screw on the case of my Virgin TiVo box. The problem immediately diminished, so I continued to daisy chain this wire to my DVD recorder and TV, and the hum has gone.

So can I leave it like this?
Click to expand...

sq225917 said:
No.
Click to expand...

The danger is that if the earth wire comes lose in the plug and happens to come into contact with the live pin (highly unlikely but still possible) then the casings of your equipment will become live. This is a perfectly safe way of doing things.
http://www.csi-products.co.uk/uk-ea...gclid=CNOjibvws9ICFTco0wodecgB-g#.WLX-0rTfWhA
 
Thanks for all the replies, much appreciated.

linnfomaniac83 said:
The danger is that if the earth wire comes lose in the plug and happens to come into contact with the live pin (highly unlikely but still possible) then the casings of your equipment will become live.
Click to expand...

This did cross my mind when I wired the plug, but doesn't the same danger apply to every single 3-pin earthed power plug that we've ever wired, or am I missing something (very likely!) ? I guess it wouldn't hurt to insulate the other two pins though.

linnfomaniac83 said:
This is a perfectly safe way of doing things.
http://www.csi-products.co.uk/uk-ea...gclid=CNOjibvws9ICFTco0wodecgB-g#.WLX-0rTfWhA
Click to expand...
I like this, might get one - thanks.

misterdog said:
Many audio products are class 11 and as such require no earth connection.
Click to expand...
I do understand (bad use of the word "properly" from me), but does it then also follow that it is dangerous to add an earth connection to such a product if it solves a problem?

misterdog said:
Note, earth and ground are not the same.
Click to expand...
This is one of life's great mysteries to me, so you can see why I came here for some advice :) I've tried and failed to understand this.

I still need to find the root cause of the ground loop, but this quick fix buys me some time unless there is a safety concern.
 
Normally (on an earthed appliance) you have three cores with an outer sheathing that the cable grip can get a decent purchase on, with a single core, you can't. Also with a correctly wired plug, the live should break away first and you'd know something was wrong because the appliance would cease to function.
 
Fair points!

Regarding the bonding plug you pointed me at, I've found these available with binding posts (better) but with a 1 Megaohm resistor. Should this resistor matter?
 
TimR said:
Regarding the bonding plug you pointed me at, I've found these available with binding posts (better) but with a 1 Megaohm resistor. Should this resistor matter?
Click to expand...

The resistor is there to protect people when those plugs are used as intended, to avoid static electricity. 1M is fine for draining static electricity, but won't work if you are trying to avoid ground loops.
 
If you are a bit unsure of the "if the wire comes loose" problematic, route your grounding wire to a dedicated separate mains plug, on which you internally isolate the "hot" plugs completely.
 
Julf said:
The resistor is there to protect people when those plugs are used as intended, to avoid static electricity. 1M is fine for draining static electricity, but won't work if you are trying to avoid ground loops.
Click to expand...

Yep, these are for use with a lead that carries the resistance. Ideal for the OP though, although he'd need to figure out how to connect hi ground lead to it. Either an ESD lead without a resistance or just soldering the wire to the stud would suffice.

topa said:
If you are a bit unsure of the "if the wire comes loose" problematic, route your grounding wire to a dedicated separate mains plug, on which you internally isolate the "hot" plugs completely.
Click to expand...

Yes that'd work, just so long as it's done properly.
 
Its a shame that "chassis" as a term has died out, although I can understand why it has. Some general comments...

One relevant meaning of the terms here means 'Earth potential". This is *assumed* to mean that everything and anything at this potential has the same number of negative charges (electrons) in it as it does positive charges (protons). (I'm ignoring other particles as I'm not a particle physicst :cool: )

The other meaning is "the reference point for '0 Volts'".

Electrostatics and safety tend to be based on ensuring things are at the "Earth potential". Signals however may be defined as having voltages (i.e. potential *differences*) relative to '0 volts'.

Not all hum comes from loops. You can get hum for various reasons. And systems vary in how a problem may arise. Thus assuming one solution always will work is unreliable. It may need experiment to determine what works best for safety and reducing unwanted hum.

BTW I learned about some of the above the hard/interesting way. Having kit on a large telescope at the top of a high, dry mountain.

There almost nothing was at "electrostatic Earth potential". And the telescope's potential used to drift up to kV levels whilst the kit all worked happily. We used to take turns to be the first person to touch the telescope if no-one had touched it for a few mins. The actual charge wasn't enough to damage you. But it woke you up. 8-]
 
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