I have a 3-phase 230V residential supply. It's divided among 3 groups of MCBs all powering single phase devices.

The problem I'm having is that whenever the air conditioning (single phase) turns on during cold start, for example after a brief power outage, the AV receiver connected to a projector loses its video signal for a few seconds.

What I've tried/verified:

  1. The AC and AVR/projector are on completely different phases and MCBs. I thought this was supposed to prevent voltage drop but something is still causing interference.
  2. The electrical and AV cables are routed on the opposite sides of the room to avoid interference.

Possible culprits:

  • The power line for the projector and AC are routed through the same conduit
  • The AVR rated for 120V is connected to a step down transformer (220/110V).

Any ideas on how to troubleshoot this?


Forgot to mention the AVR/projector is connected to a double conversion online UPS, without it the issue was much worse as both the audio and video would cut out.


The issue was caused by the long HDMI cable that connects the AVR to the projector. I haven't quite figured out what is causing the interference as the only other cables in its proximity are audio cables going from AVR to speakers. The HDMI cable works fine in normal operation and the interference only happens right after an AC cold start.


Issue has been isolated to EMI caused by the long power line of the AC (antenna effect?). Even though the power and AV cables are routed on the opposite sides of the room, the room itself is quite narrow in width.

  • \$\begingroup\$ after a brief power outage In that situation I understand if there are glitches.Anyway, you got 3 phase, and earth. If there's issues with earth between your house and outside, electricity from one phase could find an alternative way to another phase, through the common earth inside your house. \$\endgroup\$
    – MiNiMe
    Commented Oct 4, 2023 at 10:37
  • \$\begingroup\$ @MiNiMe I forgot to add the AVR/projector is connected to a double conversion online UPS. The issue was much worse before, even the audio would cut out. As for one phase passing through another, isn't that called floating neutral where the supply transformer's grounding is broken? This would blow all the small power appliances as the lowest load would see the highest voltage right? I don't have enough space in the panel to add a voltage protection relay but yeah, this would be a worst case scenario involving insurance and lawyers and whatnot. \$\endgroup\$ Commented Oct 4, 2023 at 11:35
  • \$\begingroup\$ Ok, that's bad. It goes to show that an UPS isn't the cure for every type of power disturbance (of course depends on type of UPS and other factors). You say your utility is 3-phase 230V*, wouldn't that be 400 V? \$\endgroup\$
    – MiNiMe
    Commented Oct 4, 2023 at 12:16
  • \$\begingroup\$ @MiNiMe It appears the issue is not with the power supply, but some sort of interference with the long HDMI cable connecting AVR to projector. I'm still working out how that happens as there are no power cables in its proximity. \$\endgroup\$ Commented Oct 4, 2023 at 13:58
  • 1
    \$\begingroup\$ Sounds like you have momentum towards a solution, I'm happy for you. \$\endgroup\$
    – MiNiMe
    Commented Oct 4, 2023 at 14:10

2 Answers 2


With balanced 3 phase loads, there is minimal current (and therefore voltage rise) on the neutral wire. In fact some 3 phase loads don't even have a neutral wire.

With unbalanced loads on a 3 phase supply (typical when you have individual 1 phase loads), there can be large neutral currents, which can impact the voltage rise/drop on the neutral wire (going back to the star point of the supply transformer).

This neutral impact (caused by the load on one phase), will be seen on other phases as a change in their own phase-neutral voltage.

Motor start currents can be massively higher than running current, and are a common culprit for inter-phase interference.

To diagnose further, I'd suggest you want a scope trace of the L-N voltage (and perhaps N-E voltage) on your AV phase when the aircon motor starts. That may show up the problem I'm describing, or may show something else.

(Other causes may include direct RFI interference from the aircon relay switching on, perhaps due to a failed snubber/suppressor.)

In terms of solutions, an active UPS, which always does AC-DC-AC conversion might be the easiest (if least elegant) route.


There are a couple of things that could cause that issue.

If you are sure that this is, in fact, because of voltage drop, then you should consider that although the devices are on different phases, they still share the neutral line (or null). Half of the voltage drop will be on over the null line (if the cross-section of the wires is the same; most of the time, the null is a thinner cable as the total current through it is lower than the phases). This means that if there is an increase in current consumption over one phase, the voltage of the null rises. That will affect the voltage of all the phases. So, even the load on different phases can affect each other through the shared null.

The other reason could be EMI of such big/sudden current changes. If that is the case, then better shielding for projector cables could help.

  • \$\begingroup\$ Thanks, it's interesting how the load on one phase can affect one on another. I forgot to mention the issue was much worse before I added a double conversion online UPS for the AVR/projector. So it can be fairly positively concluded that this is in fact an electrical issue right? Would running a new neutral line just for the AC fix it, also do you know any easier way? \$\endgroup\$ Commented Oct 4, 2023 at 11:23
  • \$\begingroup\$ Well, if using the UPS solved the issue, it proves that the issue is from the power line. However, UPS also filters a lot of noise from the power input. For voltage drop on other phases, having a separate null from the source could help (especially if it's a long wire). But if the problem is fixed by UPS because of rejecting the noise from the power line, maybe a common mode rejection transformer on the power input of the projector could help. I can't think of anything else at the moment. \$\endgroup\$
    – Saadat
    Commented Oct 4, 2023 at 11:35
  • \$\begingroup\$ The issue was improved with the use of the UPS but not entirely solved as the video is still cutting out. \$\endgroup\$ Commented Oct 4, 2023 at 11:55
  • \$\begingroup\$ In that case, it could be EMI noise over video cable. What type of cable is it? Can you try with shorter (only for test) and better cables? \$\endgroup\$
    – Saadat
    Commented Oct 4, 2023 at 11:58
  • 1
    \$\begingroup\$ Glad you found the issue. The problem with HDMI cables is that they are expensive. So, the manufacturers usually design them for general usage. For a normal environment where there isn't much noise, that cable works, and that's enough for the manufacturer. If you have to use a long cable, you might find a more expensive cable with the same length that can reject the noise, or get two shorter high-quality cables and use an HDMI repeater to solve the problem. \$\endgroup\$
    – Saadat
    Commented Oct 4, 2023 at 13:51

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