Macca's System Blog

[jammy395]

I take it your not a Gambling Man then Bill.

Mmmmm will I, wont I.

[BilliumB]

I think farmers are probably quite big gamblers! Every harvest is a gamble.

[Dr Bunsen Honeydew]

The BMU can be used with *ANY* hi-fi.

[BilliumB]

So, does that mean the mains earth is connected through?

[jammy395]

[Dr Bunsen Honeydew]

Throughout what. Again you don't understand either Class II or balanced mains. Balanced mains is plus 12Ov - 0v - minus 120v. Plus 120v is on the positive pin of the socket and obviously any plug in it, minus 120v is on the negative pin of the socket / plug, and the earth pin of the socket / plug is connected to the balancing transformers secondary coil centre tap i.e. 0v. ALL balanced mains systems do this not just Class II NVA BMU.

I really do not understand what you are asking me, if you are asking is it safe to use with Class I equipment then the answer is yes, any over voltage on one of the legs or sudden current drain on either leg will trip your RCD or fuse in your consumer unit, and any current leakage on to the case of a hi-fi unit will trip it as well. Your consumer unit reacts far more quickly than the stupid waste of time fuses fitted to our stupid BS 1363 13amp standard plugs now used.

If that doesn't answer your question then give up asking.

[Dr Bunsen Honeydew]

Balancing or partial balancing is used in many power applications, and with DC as well. A classic example is train power systems. In Southern England the pre nationalisation Southern Railway adopted a third rail electrical supply at plus 650vDC (being converted at the moment to plus 750vDC). So three rails, the outer one carry power that is picked up by what they call a shoe rubbing along it. Now look at the tubes in London they have 4 rails, same 3 as the old Southern system but one more in the centre of the track - so why? Well tube tunnels are metal and leakage current from the full 630vDC would contaminate the tunnel walls so they run a split rail partial balance. The outer rail carries plus 420vDC and the centre rail carries minus 210vDC, which really helps to stop the leakage.

Leakage current is also a problem with all hi-fi electronics, but more so with Class I products. Balanced mains cancels out this problem, which is why balanced mains drops the noise floor of your system so much and allows greater information retrieval which = more music.

[BilliumB]

I seem to be winding you up - I can honestly say that this is not my intention, so sorry if this is the result; I've spent many interesting hours reading through your very informative posts (on many subjects) on the forum, the archives and the helpful links - and even though you might feel otherwise, I'm not stupid and do understand what we're talking about.

Anyway, thanks, yes I think you have answered my question which I thought was quite simple (and I quote my earlier question), whether the BMU "provides continuity of earth connection from the wall socket through to the earth pins on it's faceplate". From what you say I infer that the answer is NO.

I fully understand what a balanced transformer is (o/p -120, 0, +120) and why it's of benefit, and thought that within the BMU you probably simply connect the 'earth pins' of the output sockets to the centre tap of the transformer secondary (so they can all float up and down together) and not also directly (with a simple bit of wire) back to the wall plug earth pin. Thanks very much for clarifying this for me.

I have no doubt that you're right about fuses, RCDs, safety etc - can't remember when a fuse last blew.

Thanks very much for taking the time to discuss this.

Cheers. Bill

[Dr Bunsen Honeydew]

I haven't answered your question I have tried to explain why your question is meaningless.

[Dr Bunsen Honeydew]

This is an interesting article more from the computer world than hi-fi, but talks about leakage current. Now leakage current is not fault or short circuit related, it is current inducted into a metal case.

"When a voltage is applied in a circuit the electric current starts flowing (or the field establishes itself). Now in an AC home circuit the current flows through the circuit like in DC, but happens to reverse directions 50 or 60 times a second too (Hz).

So why do some appliances have electric current leak onto their metallic surfaces at all in the first place. Shouldn't all appliances's internals be designed so that there can never (or rarely) be any leakage of current ?

The whole point of my question is that why do we blame the lack of grounding when an appliance gives an electric shock - isn't the appliance equally to blame for being designed in a way that allows for charge leakage ?

Hence in the event of an electric shock isn't it equally important to investigate the appliance (in this case it's actually a custom assembled desktop computer) to find out why its circuit is leaking charge to its metallic body parts, instead of always expecting the grounding to remove that excess charge to the earth.

Another way to paraphrase this question is - are some appliances (especially assembled computers) likely/expected to leak charge. Hence in the event of rare shocks sometimes shouldn't it be more important to investigate the appliance itself for having the proclivity to receive leaked charge instead of blindly checking the grounding"

Change the word computer to amplifier and it would still be the same argument.