View Full Version : SAFETY ALERT, Perspex cased BMU (Balanced mains unit)
Firebottle
05-07-2021, 15:22
I have found a serious safety problem with an encased BMU. DO NOT OVERLOAD YOUR BMU, this applies equally to any transformer.
The unit in question is a perspex cased BMU, I stress that this transformer has been subject to OVERLOAD.
The enclosed 1kVA transformer was held in place with glue and expanded foam packing, with no ventilation or method of heat transfer whatsoever.
Every power transformer has a specified temperature rise (above ambient) at full rating, usually 55 degrees Centigrade.
Think about that.. 55 degrees above ambient. Usually a transformer has free air around it that conducts heat away from it.
This is what I found:
https://i.imgur.com/F0qCE88l.jpg
I don't know what loading had been used on this BMU but the transformer became so hot that (1) the foam packing has melted, plus most worryingly (2) the transformer insulation and the input wiring PVC insulation has melted, causing a short circuit. The input wiring was held against the top of the transformer by the frankly shocking construction employed.
The unit was causing a trip to operate in the consumer unit (mains supply).
https://i.imgur.com/eUemtGUm.jpg
The melting point of PVC insulation is >140 degrees C, so not a great situation.
Unfortunately due to the construction method a visual inspection of similar units is nearly impossible.
The minimum advice for anyone using one of these would be to operate it at nowhere near maximum rating.
Wakefield Turntables
05-07-2021, 16:55
WTF! :wowzer:
hifinutt
05-07-2021, 18:53
thanks so much for highlighting , nearly bought one of these in past .
Beobloke
05-07-2021, 19:15
Shocking (sorry…) ;)
Frankly I’m not surprised. I’ve seen pictures of the inside of one of his ‘Statement’ products and it was appalling.
southall-1998_mk2
05-07-2021, 19:28
Jesus fucking wept! You could destroy earth with that thing.
S.
Radford Revival
05-07-2021, 20:15
Oops, wrong kind of insulation :D
Firebottle
06-07-2021, 09:10
Having received feedback from the manufacturer it is confirmed that the design principle is one of containment under fault conditions.
It is considered not to present a fire hazard but my advice of under-running the unit stands.
Beobloke
06-07-2021, 09:37
Looking at the pictures, my advice would be to not plug it in under any circumstances!
legb4rsk
06-07-2021, 10:09
Having received feedback from the manufacturer it is confirmed that the design principle is one of containment under fault conditions.
It is considered not to present a fire hazard but my advice of under-running the unit stands.
Sounds very familiar. cladding?
Firebottle
06-07-2021, 10:40
A statement from the manufacture:
'Our current BMU comes with the option of a thermal cut out and we speak to each potential purchaser about the calculations required to check the power consumption of the equipment that they want to use the BMU with. The BMU is rated at 1Kva. We recommend that you do not use beyond 70% of that capacity. If we are not satisfied that the BMU will be used within that limit, we won't sell it to you.'
Bear this in mind if you buy one second hand.
Just for contrast:
Sure they are doing things differently now but that's just unsafe.
https://i.imgur.com/F0qCE88l.jpg
https://i.imgur.com/gqDiuI6.jpg
You ought to "name names", though I'm pretty certain who the manufacturer is/was.
IMO, all BMUs should have a ganged double-pole circuit breaker fitted to the secondary winding.
Just for contrast:
Sure they are doing things differently now but that's just unsafe.
https://i.imgur.com/gqDiuI6.jpg
Is that yours Gary? Looks like it is fitted with suitable MCBs, as discussed above. :)
Is that yours Gary? Looks like it is fitted with suitable MCBs, as discussed above. :)
Yeah,
That's mine Barry,
My 2nd BMU unit, hardwired big 10kva Mutha!
Took advice about BMU safety before setting it up from a mate who deals with electrical safety professionally, and yup, twin pole RCBOs all round.
That unit Alan showed should serve as a warning, things could have really got out of hand dangerous.
It's good the Company has adressed concerns, hopefully to (maybe?) rethinking their design, some of the wiring look spliced with heatshrink?
:hmm:
Ali Tait
08-07-2021, 06:43
You ought to "name names", though I'm pretty certain who the manufacturer is/was.
IMO, all BMUs should have a ganged double-pole circuit breaker fitted to the secondary winding.
I built mine that way, MCB on the primary, RCBO on secondary, both double pole.
Lawrence001
08-07-2021, 09:58
You ought to "name names", though I'm pretty certain who the manufacturer is/was.
IMO, all BMUs should have a ganged double-pole circuit breaker fitted to the secondary winding.I think Alan initially had but there was a discussion on another forum and it seems to have gone now.
I built mine that way, MCB on the primary, RCBO on secondary, both double pole.
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.
earsopen
08-07-2021, 15:40
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.
i think for legal reasons the name has been removed .
Surely the onboard fuse would protect the BMU from overloading unless the fuse had been bypassed or replaced with a higher rated fuse?
Ali Tait
08-07-2021, 18:12
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
08-07-2021, 19:29
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
08-07-2021, 19:42
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
08-07-2021, 22:09
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
08-07-2021, 22:10
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
09-07-2021, 04:36
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.
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.
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