SAFETY ALERT, Perspex cased BMU (Balanced mains unit)

[Ali Tait]

Yes - that is correct. I should have said the circuit breaker on the secondary side should be a ganged double-pole RCBO.

Since both sides of the secondary are +/- 115V with respect to earth, under certain fault conditions there could be a leakage current to earth on one side, but not the other, with the leakage current sufficiently low so as not to trip any safety device on the primary side (such as any fuse in the plug top, or device in the consumer unit).

Yes my thoughts at the time also. Mine is in a metal case though.

[Beobloke]

So, all of the rest of us who do not know the manufacturers name of the BMU photographed, are left quaking in our boots because the BMU we are using might catch fire and burn the house down?
Some may make assumptions about the manufacturer or retailer, but how do we get to know for a fact?
This does need more explanation.

If you type “Balanced Mains Acrylic” into Google, the first result will tell you all you need to know.

It should be clarified that the pictured unit is an old one. I would hope that the manufacturer may well now have gained at least a basic grasp of electrical safety.

[Firebottle]

Surely the onboard fuse would protect the BMU from overloading unless the fuse had been bypassed or replaced with a higher rated fuse?

A large toroidal tranformer has a high inrush current that needs a high value fuse fitted in order to not blow on switch on.

A much more acceptable approach is to fit some sort of surge limiter and a lower value of fuse.

[Blackdog]

Yes - that is correct. I should have said the circuit breaker on the secondary side should be a ganged double-pole RCBO.

Since both sides of the secondary are +/- 115V with respect to earth, under certain fault conditions there could be a leakage current to earth on one side, but not the other, with the leakage current sufficiently low so as not to trip any safety device on the primary side (such as any fuse in the plug top, or device in the consumer unit).

This was something the designer failed to understand. He simply claimed that shorting one half of the balanced output to earth, caused the circuit breaker to trip in the consumer unit; something which it would do anyway, as the primary current would be very high.

Not sure about the UK, but here in Canada BMU's must have GFCI's installed as well as primary and secondary breakers or fuses. This particular unit doesn't pass the most basic safety parameters.

[Lawrence001]

My old MF Studio T amp used to trip the mains every time I switched it on. I had to leave the amp switch on after it tripped, go to the fuse box and force the switch up for a few seconds for the surge to pass. Not an ideal situation by any means!

[Ali Tait]

A large toroidal tranformer has a high inrush current that needs a high value fuse fitted in order to not blow on switch on.

A much more acceptable approach is to fit some sort of surge limiter and a lower value of fuse.

Yes, I fitted a large NTC and DC blocker to mine- as it’s a 3kva transformer, it kept tripping the CU.

[Mr. C]

It that had a CE marking, then trading standards is the place to go. No way in a month of Sundays would that ever pass a complienece test!
With transformers over 600Va a soft start is very benefical. When we have construct these units for customers, we first measure the inrush current under various loads. The we use a programmable soft device with the matching current profile, along with a full dual differenical a/c filtration circuit and a custom dc blocker. Usullay a 5Kva BP Tx along with the correct value RCD and MCB depending on circuit configuration.