BMU Safety issues

[Ali Tait]

I made mine to be portable, and to be safe to use whether the kit is double insulated or not (most is anyway).

The reason I have fitted a double pole RCBO on the secondary is that it is possible that a fault could occur on one secondary leg, but the other could still be live, for instance making the chassis of a bit of(non double insulated) kit live. I appreciate this is pretty unlikely, but I prefer to err on the side of safety, having worked in the high voltage industry for coming on 30 years now, and been to seven funereals of work collegues, some killed by some unlikely circumstances, I prefer the belt and braces approach. That's my choice, I couldn't care less what approach others wish to take. In any case, they are easy to fit, and I doubt I could hear a difference in SQ with and without, so why not fit them anyway, also you don't know what kit you may buy in the future.

Mine has been on pretty much 24/7 for coming on two years now without any issues.

[Ali Tait]

A good article here on the safety aspects-

http://www.soundonsound.com/sos/nov0...stalk_1106.htm

[Oldpinkman]

Thanks Ali. I think there was some confusion (certainly in my head) reading other posts as some of these power supplies seem to be designed for double insulated only equipment and are a pure isolating (from earth) transformer albeit with +115/-115 balanced output and do not have the centre tapping on the secondary tied to earth and connected to the outlet sockets which is different from yours. Might build one of these myself as all my equipment is double insulated. In this type any type of RCD on the secondary would be pointless.

I don't think there is any need further protective devices (MCB or fuses) on the secondary side as there are the fuses in the 13A plugs supplying the equipment and in the back of the equipment itself and any faults in the transformer secondary winding/wiring would be seen and interrupted by the mains supply fuse on the primary, I think

I haven't looked at every device, but the ones I have are NOT isolating - that is they do not have a floating ground, and connect ground to true earth

The fuse in your 13 amp plug won't cut the power on the neutral wire. If there is a fault that blows a fuse, there will still be 110v inside the equipment. THAT is the point about double pole switches

A dead short on the secondary ( a bit of copper shorting it) would induce such a high current in the primary that it would blow the mains supply fuse (surely these days you have circuit breaker trips). A resistive short (my incandescent bulb, a human body) might not. A 1kohm short on a 110v supply would produce only 110 milliamps of current - why would that blow a fuse?

[Oldpinkman]

A good article here on the safety aspects-

http://www.soundonsound.com/sos/nov0...stalk_1106.htm

Unbelievable given the research I have done into this that I haven't seen that article before. Nice one Ali!

[Ali Tait]

Pretty sure I've sent you that link before.. :-)

Anyway, it's a clear explanation.

[Oldpinkman]

Pretty sure I've sent you that link before.. :-)

Anyway, it's a clear explanation.

I think you're probably right

Probably premature senility - I think I get it from my Dad. It looks familiar now - although I think I must have only skim read it. Thanks anyway

[Ali Tait]

No worries. :-)

[JohnJo]

I see now that there is a noise cancelling function of the centre tapped earthed secondary which obviously wouldn't be there in a basic isolating tx circuit.

So, Ali's is the optimum circuit then and his use of double pole RCBO is spot on. Ali I like your belt and braces approach with electrical safety.

Very informative thread (for me) Thanks to all

[Oldpinkman]

I see now that there is a noise cancelling function of the centre tapped earthed secondary which obviously wouldn't be there in a basic isolating tx circuit.

So, Ali's is the optimum circuit then and his use of double pole RCBO is spot on. Ali I like your belt and braces approach with electrical safety.

Very informative thread (for me) Thanks to all

Johnjo - be careful about this noise cancelling. It is probably of little use in a domestic situation. THE DEVICE DOES NOT CANCEL NOISE ON THE MAINS. It cancels any extra noise added to the mains on the lead between the BPS and the HiFi appliance ONLY. However, not only is that noise likely to be insignificant, if it exists at all, the same noise would be added to the lead from the BPS to the wall socket AND NOT CANCELLED BY THE BPS. So the net gain is NIL. In the unlikely event that there is any environmental noise to be added to a cable in the room, without a BPS it will be added to the cable from the Hifi to the wall, and with a BPS IDENTICAL NOISE would be added to the cable from the BPS to the wall AND NOT CANCELLED.

This is of potential value in professional studios where you can have a short cable from the BPS to the wall, but long cable runs from the BPS to the appliance - but it is not the main reason for their use. Personally (having made the same mistake until it was carefully explained to me) I think people get hold of the wrong end of the stick about balanced connections and noise cancellation.

It is identical to a balanced signal connection. The balanced connection prevents noise being added to the signal cable - IT DOES NOT REMOVE ANY NOISE FROM THE INPUT TO THAT CABLE. This is far more valuable with signal cables where the signals are very low level, the cable runs are long, and the environmental noise potentially proportionately quite high. It is arguable whether it is of any real benefit in the context of a mains cable - but if it is, it would only be so in the context of VERY long cables to the equipment, relative to a short cable to the BPS.

And it has nothing to do with the way noise cancelling headphones work (it happens the "mate" I refer to was one of the early pioneers of noise cancelling at Essex Uni, subsequently developing commercial systems in Arizona)

Hope that helps even more

[Ali Tait]

It doesn't change the fact that it seems to make a real improvement to the sound of a system. In Gary's system, it totally transformed the sound, an amazing improvement akin to plugging in a totally different system, so they certainly do something!