Device X Blows Device Y Fuse

[jshailes]

This evening I realised my subwoofer wasn't working and after a little investigation discovered it's internal and power cable fuses had blown. After replacing them I turned it on and off, all seemed OK. I then plugged in the phono signal cable and turned it on and instantly the fuses popped again.

At the same time the tv turned off and began flashing a red power light indicating an error. Unplugging the power to the tv and plugging it back in and it showed no sign on life. I replaced the tv power cable fuse and it's back to blinking power led so I assume some internal hardware problem.

How could powering on a device on one side of the room affect another device plugged in elsewhere in the room?

 

[Wilko]

Firstly, those fuses in the subwoofer didn't blow just for fun, and they've done it twice. Please may I suggest you stop, as you have done all you can. There is clearly a fault and it could be dangerous for you to proceed. Could be either unit, and there is a slim possibility the audio connection lead may be live. Really, you need a technician with test kit to diagnose and advise. Cheers, David.

 

[jshailes]

Thanks for the advice David, we will likely be getting an electrician out.

In terms of the science behind how one device could affect another, can you offer any explanation?

 

[Lucien Nunes]

Two possibilities come to mind. One is that the sequence of working/not working after your fuse replacement was coincidental, and in fact both units were damaged in succession by an external cause, such as severe mains transients or overvoltage. If you have a normal single-phase supply where everything runs from the same line and neutral in the consumer unit, then you would likely have noticed this problem in other ways e.g. other items damaged or flickering lights. Factors such as overhead supply and electrical storm would point towards this cause.

If, instead, the root cause was within the subwoofer, it might be that part of its circuitry at mains voltage has come into contact with the audio input. In equipment from reputable manufacturers this is extremely rare and usually indicates catastrophic internal damage. For example, if a product is dropped and the transformer breaks free inside due to the impact, it can drag mains-voltage wiring into contact with other parts. Because this is a dangerous scenario, equipment (especially class II / double insulated equipment) is designed specifically to minimise the chance of this happening. Other causes of such a fault might be overheating, incorrect manufacture, water ingress or incompetent prior repair.

What might then have happened is that after you replaced the fuse with the fault still present, when powered up while connected to the TV, current passed along the signal cable, through parts of the TV internal wiring and circuitry and to earth via some route (if the TV itself is not earthed, then through some other attached device or aerial cable etc.) If certain critical parts of the TV circuit, such as the power supply regulation circuit were damaged by the fault current from the subwoofer, then the TV's power supply may have malfunctioned and blown its own fuse. I.e. it might not have been the fault current from the subwoofer that directly blew the TV fuse but a chain-reaction from the damage.

Both items need autopsy by a competent electronic service technician to determine the sequence of events. If the subwoofer fault was due to faulty construction it should be reported to the manufacturer as other units may be affected and could pose a shock risk. If due to damage, then you are lucky the result was only a damaged TV.

 

[Wilko]

Hi - well covered by LN !
Continuing the safety theme - please confirm you have RCD protection on all your power ccts at least. If not, then many here can help. Just post in the 'seeking domestic electrician' forum. All the best, David.

 

[jshailes]

Hi Lucian, thanks very much for your response, very informative.

Just a quick note about the interconnectivity - the subwoofer connects via phono to an amp and then hdmi from the amp to tv. Given this, do you still think this is a likely cause? The sub has been transported recently although I would say it was done very carefully, and certainly wasn't dropped. The signal cable runs in the wall cavity - if I'd drilled through signal/electric cable could this explain to problem?

Is there any further investigation I could do myself that would narrow down the cause? I was thinking of plugging in a different amp and running a fresh cable between the amp and the sub and seeing if it still blows the sub fuse. If it does then that singles out the sub as the problem. Do you think it's risky in doing so?

You mention an 'electronic service technician' - is that the same as an electrician or someone that just specialises in electronics?

 

[Lucien Nunes]

The presence of the AV amp doesn't change much, especially if it is Class II i.e. not itself earthed. The only additional factor is that it might now be compromised too.

Drilling through the cables and shorting the signal cable to a power cable without tripping the power circuit protection seems improbable. It would then have had to cause specific damage in the TV and subwoofer to destroy both their power supplies in a way that would then blow the fuse. If the subwoofer is Class II and has only the one signal cable, this is almost impossible and I would rule it out.

Do not carry out further tests by powering things up - fuses are there to reduce likelihood of fire, not as diagnostic aids. You are at risk of electric shock or fire, causing more damage to equipment or obliterating the evidence of the original fault.

By electronic tech I mean latter-day 'TV repair man' who will understand the internal circuitry, which is a different skill set to electrical installation. There are a few (including some members here) whose experience spans both domains which is obviously an advantage when tracing electrical safety issues in electronic devices.