Fuse box buzzing

Smoke marks would not be a good sign alright, I am not an electrician, but I have heard boxes buzzing when showers are operating - particularly the plastic types like you have shown. It is 50hz mains hum vibrating on the fuse box case. Just tightening or loosening a cover screw will often stop the case resonating. You will still hear a normal low hum, caused by the heavy load going through the shower circuit, but the buzz, is the fuse box case vibrating in resonance.

The smoke marks would need to be looked at though - might just be heat dis-colouring dust, but better to get it checked. There is a natural build up of heat on parts of the fuse box (over heavy load circuits), but it should never be enough to actually smoke.

That's a priority unit to ensure that only one shower can be used at a time to avoid overloading the incoming ESB supply cables. The two contactors on the right of the picture provide power to the two showers, but are interlocked so that only one can be energised at a time. The buzz is likely to be coming from whichever contactor is on and is fairly normal. The black marks look like dust to me, the contactors run slightly warm due to losses in the coils, which creates a convection current and deposits dust from the air as the warm air passes over the outside of the CU. Replacing the contactors would probably quiet it down for a while, alternatively if it's keeping you awake at night you could just about fit a small timer to the panel to break the control circuit at night so both contactors would be off. Neither shower would work though unless the timer is on.

If you just have a pumped shower you shouldn't need any of this complexity, this kit if for 'electric' shower which heat the water and have huge power demand so running two at a time would blow the main fuse.

For a boost/pumped shower a normal switched 3amp spur for each with 30mA RCD protection at the main board would be fine, no issue running both at the same time...

I looks like you did in post 12. Change it for a proper one you said. To me it looks like dust, not smut/smoke marks, but closer inspection needed. More evidence needed anyway before suggesting swapping boards and elecricians.

So you did in fact recommend changing the board, based on the OPs own diagnosis. Then talk about the standard of tradesmen giving a diagnosis without seeing the board...

The boards consist of 2 contactors, current sensors, and mcb/RCD. What do you think the buzzing could be amongst those items that only happens in poor quality boards? And how does "smut" only occur in said boards? And how did this smut/smoke get on the front surface of the board, but little or none on the angled surface above the devices... Surely that should be black....

I'm delighted to see this post as I've just had a unit fitted in my house as I've installed another electric shower and it buzzed constantly over the last few days, I had my electrician back and he checked everything and could find no fault but he agreed that it was not satisfactory for it to buzz continuously. He called the manufacturer's technical rep to be told that Hager had recently changed some materials in their shower non-priority units which had caused the buzzing noise, they agreed to replace the unit with the original contact material FOC and its now on its way to my electrician. In the interim I turn it off at night until its replaced.

No. There is more for you to inspect there now. Take your time now...

We have a board that might be fine, or might not be. But you recommend changing entire board.... Then say you didn't..

Which is it?