BMU Safety issues

[Oldpinkman]

EDIT : There appears to be no member facility to amend the title - mods, feel free
A BMU is a Balanced Mains Unit. It is a device to supply "different" mains - and in this context is for your to plug your HiFi mains plugs into. It comprises a transformer with a centre tap, which changes the normal mains supply of 0v neutral 220v live into minus 120v neutral plus 120v live.
END EDIT

Given the context in which the misunderstandings about this subject have been aired on HiFi forum, it is quite difficult to make the important point about safety without offending.

The issue regarding BMU's is not manufacturer specific.

Nor are they "dangerous"****

**** Let me heavily qualify that comment. Fireworks are not dangerous. If properly used, and properly understood.

Neither Fireworks nor BMU's are prohibited from sale or illegal. They are therefore not considered "too dangerous". However one wouldn't want to make a comment like

"A firework rocket is potentially dangerous, and should never be lit whilst held in the hand" only to have a manufacturer percieve that as an attack on their firework because they have recently launched one and make a statement in response like "it is perfectly safe to hold a rocket in your hand while lighting it. Fireworks are not illegal"

It is very obvious from reading comments on HiFi fora that BMU's are not well understood. There is that "black art" subjectivist element to them. They have a useful purpose, and can achieve a worthwhile result, but they start to acquire "magic" properties in the minds of some.

My limited expertise in this subject derives from an interest in buying one - and is the product of asking a good friend, who understands the subject properly, to explain it. I had a hum issue - earth loop of course - on the AV input to my home system only. I think a BMU might have fixed it - although in the end we fixed it at source (diferential ground on the satelite dish cable). And the BMU may have produced additional benefits by eliminating ground loop interference which was not obviously audible, but none-the-less present on the other inputs.

I looked at a BMU made by a reputable manufacturer - which retails for £340 online. It carried a number of warnings regarding its use on its web site. These included

1. Best used on a separate spur and not a ring
2. Can be used to supply incandescent lighting supplies but should not be used with other lighting supplies
3. Should not be used on a spur which also supplies lighting controllers
4. Should not be used to supply other mains conditioning products
5. The balance power supply should be used with a double pole switch to ensure that the equipment is fully isolated when the power is in the off position.

There was also a reference to not turning on within one minute of turning off, but this was because this supply was a bit more sophisticated than a few sockets on a transformer, and included a soft start device to avoid nuisance tripping of circuit breakers due to surge currents. (It also has surge protection and EMI/RFI filters incorporated - you must have spotted it by now!)

To shorten the story a bit, I decided it was not for me, partly because the original earth loop problem had been resolved, and partly because of some of the restrictions regarding my domestic arrangements (I wasn't prepared to start putting in dedicated spurs at this stage in the game). However, partly also as a result of my curiosity being piqued by some robust and authoratative statements being made which were counter-inutitive given my knowledge of RCD's and electromagnetism, I got my friend to thoroughly explain the subject, and address specific points raised. I learned the following points, which I think are worth sharing with others possibly interested in these devices

1. The reference in the warning by the manufacturer of the device referred to above, to the need for a double pole switch is because both "live" and "neutral" are "live" on a mains lead plugged into a BMU and a single pole switch might not turn off power to the hifi component. Because most hifi is made to plug into conventional domestic unbalanced mains, most hifi equipment does not have a double pole switch for its mains
2. A lot of hifi has fuse protection. This protection is only on the "live" rail. It will not blow and cut the current supplied on the neutral rail - because when used with normal unbalanced mains the neutral rail does not carry a voltage. This may have important implications for a Class1 device which relies on an earth connection to the case. A short of the live connection to the casework will blow the fuse. But with a balanced connection a short of the neutral rail (now carrying 110v) will not blow a fuse (although it may blow a circuit breaker on the supply to the BMU if it is a dead short). The practice adopted by some forum members building their own BMU's of fitting RCBO's to both output rails addresses this issue buy adding a "fuse" to the "neutral" supply
3. Domestic RCD protection will not be tripped by a short to earth on any cable or device plugged into a BMU. This is not necessarily dangerous, but it is the case. It is not hard to prove if you find the maths and physics too hard to understand (I could explain them if asked, since I learned them thoroughly). what is needed is a short to earth with a resistance high enough not to trip a circuit breaker or fuse. A lamp (inspection lamp, table lamp) is perfect. disconnect the earth wire, and connect neutral to earth. this will trip the rcd on the house plugged in to the mains direct. It should not trip the house rcd plugged into a BMU. Only use a standard incandescent bulb for this purpose. Don't play with mains if you are anything other than certain that you are safe and know what you are doing
4. BMU's do NOT clean the mains. They do not in anyway remove the dirt from the incoming supply. Any noise on the incoming supply will be repeated on the output supply. They do offer other benefits such as preventing further noise being added to their output supply, and potentially removing earth loop issues.

They are not illegal. They are not inappropriately dangerous. They can have a useful purpose. But they should be properly understood, as a failure to understand them might be dangerous. And making factually inaccurate statements about them is very unhelpful, and potentially dangerous.

[Joe]

Note to self: Must remember to ask my electrician to go over my accounts with me.

[walpurgis]

An explanation of what the initials BMU stand for may assist members. Also a description of what the item is, what it is intended for and where it may be of use to Hi-Fi users. Not everybody is automatically 'clued up'!

[prestonchipfryer]

What is a BMU?

[Mr Kipling]

What is a BMU?

I thought it was a film following the exploits of three individuals questing for buried treasure:

The Bad, The Mad And The Ugly

MAJOR PUBLIC HEALTH WARNING:

ON NO ACCOUNT, REPEAT ON NO ACCOUNT, WHATSOEVER, NEVER EVER, EVER, bend down to pick up teacake fallen from the shelf at your local supermarket without wearing the appropriate PPE (Personal Protective Equipment). Doing so, could result in the currant shooting up your arm.

I don't quite understand why such a device is now being seen as a potential death-trap. It's not exactly a radical new concept. Or why this thread has managed to get to 10 pages…

[take5]

I think its an anagram for a predatory type of person, commonly found in the Hi Fi industry.

Not certain though.

[Oldpinkman]

I have edited my first post to the extent I can. To specifically answer "what is a BMU"

BMU stands for "Balanced Mains Unit". It is a transformer which turns normal unbalanced 220v mains into a different form of 220v mains (I am using 220v to stand for 220-240v for simplicity. The device does not voltage regulate - it voltage splits)

Instead of having live earth neutral at +240v 0v 0v it has those connections as +110v 0v -110v. The devices are widely used in the professional music market to deal with some problems caused by mains. Although those problems are more likely to be an issue in an electrically noisy environment like a studio, they can provide similar benefits in a domestic hifi installation

They are therefore sold to hifi users as a way of improving the mains supplied to equipment - and they can offer some benefits in that respect. My perception is they are viewed a bit like a "black box mains cleaner" - which supplies identical mains as the domestic socket, but cleaner. And they are very different from that. Accordingly, I think it is important to understand some of those differences.

Hope that helps

[The Black Adder]

Balanced Mains Unit..?

[Sovereign]

I also run a balance power supply, there is no inbuilt protection, no inrush circuit, no DC blocker not even an on / off switch. My hifi has a dedicated consumer unit with RCD protection just a few feet away. As it is only me that touches my hifi and therefor no one is going to be touching either the live or the neutral I consider it to be very safe, further to that I turn everything off after each hifi session so we are doubly safe.
Further still my hifi room is a detached brick building so if it does go up like the clappers I won't even wake up

[JohnJo]

BMUs: Would somebody please post a schematic for these showing earthing arrangements? Thanks.