Hopefully this isn't a totally stupid question, but my knowledge in this area is non-existent.

I have a 10 year old UPS (it's an APC 3000W 2U rack mount, thus has two fairly big transformers inside) and it's developed the typical transformer hum that's become rather loud.

I've done a bit of reading (for example) but replacing or fixing the transformers doesn't seem possible.

However, I noticed while taking the system apart that with the top case removed, the hum/buzzing disappears completely. Then slowly moving the top case back down, without touching anything, the hum will start again, and get louder as the case gets closer. It maybe starts about an inch above.

What little understanding I have is that this will be caused by the magnetic field given off from the transformers? From the 50Hz AC? Or at least I hope it's a normal byproduct because that leads me onto my main question...

I was thinking of replacing the top cover of the UPS with Plexiglas. Is this a stupid idea? Ignore the fact it'll be structurally weaker, the UPS will be in a locked rack so I'm not worried about that. What I do want to know is whether the lack of top metal cover would have any disastrous effects for the rest of the equipment in the rack? If the top case can experience vibrations and is some way shielding the internals, would this effect just be transferred to the next item above it (like a server with hard drives...) if the top case is removed? I was under the impression transformers are supposed to keep everything contained, but my buzzing case seems to contradict that?

If you want any details/photos of anything mentioned here, I can provide.

Many thanks!

  • 2
    \$\begingroup\$ What's above it in the rack? You've seen how the field strength reduces with height ... quite fast. If you can leave 2U or 4U free space, anything above that will have to be quite sensitive to be affected (especially if it has its own steel case). Alternatively, replace the top panel with a dome (top hat) of folded steel sheet to give you both shielding and spacing. \$\endgroup\$
    – user16324
    Commented Jan 18, 2017 at 13:12
  • \$\begingroup\$ Thanks. I was actually going to leave at least 2U anyway in case I expanded the UPS in future. After that it's regular Dell servers. I understand HDDs are quite well shielded/not easily annoyed by magnets on the outside. But I had to ask in case there's other factors at play here! \$\endgroup\$
    – bnx
    Commented Jan 18, 2017 at 13:20
  • \$\begingroup\$ You could try contacting APC - they would have heard of this problem if a few people experience it. There might be a simple cure like tightening some bolt or other. \$\endgroup\$ Commented Jan 18, 2017 at 19:48

2 Answers 2


The hum is usually caused by magnetorestriction of the transformer laminations.
Over time the pressure on the laminations will reduce, and you can get oxidation between the laminations allowing them to move relative to one another easier (Hum) than when new.
The transformer also has an external magnetic field and your top cover represents a magnetic short which will alter the field concentrations and paths slightly. When you take the top away the field lines move and the magnetorestrictive field paths move too. This movement now impacts slightly different spots in the laminations, so 'Hum' may actually increase or decrease. Perhaps you could read this as an introduction.

I've seen various techniques used to reduce hum.

  • Audio transformers used to have strong steel clamps on the ('E' and 'I' side laminations) along with a front and back cover.
  • Copper covers used to encase transformers and short the external field.
  • Complete vacuum potting to restrict laminations and bobbin wires moving and to suppress noise transmission

In your case you might be able to impact (reduce) the noise by replacing the fiber, hard rubber or like washers used in the lamination clamps as they normally spread (flow) with time and reduce the pressure on the laminations.

Replacing the top cover with a non-magnetic material may buy you some more quiet time, but invariably the Hum will eventually return.


In my experience the case top is there to do two things. Firstly to direct airflow if there are fans and control active cooling. Secondly to stop things being dropped into the case that can produce short circuits or other spectacular events.

The magnetic screening effect of a thin sheet of galvanized steel is almost nothing. The electric field shielding is minimal unless it is firmly grounded around the periphery with a "copper finger" gasket.

My concern with Plexiglas would be its fire resistance and high temperature performance. Perhaps an aluminum top which still has magnetic effects but not so extreme would be a good compromise.

  • \$\begingroup\$ Grounding does not eliminate magnetic fields. \$\endgroup\$ Commented Jan 19, 2017 at 3:28

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