Inrush current when turning amplifier on is causing RCBO to trip

Question

TL;DR - when turning on an amplifier, it trips the RCBO. Might it be related to the fact I'm using a smart switch? Or the wrong type of RCBO? Or can I introduce an inrush current device? How can I resolve it?

Longer version

I have a server rack in our home, fed by a dedicated power circuit with 20A RCBO, type B.

• I have two amplifiers (Monoprice 6 Zone 12 channel Home Audio Amplifier, user manual here, specs on page 28).
• Each one eats 50W on standby.
• I spend most of my time 4 floors up from where they are located, and so I have installed smart switches (Sonoff S26 UK) to power each one on remotely using an automation.
• Intermittently, when powering them on remotely, it trips the RCBO.

It may be tripping regardless of the smart switches, but it's more difficult to test. Should I just go down the stairs and manually turn them on and off repeatedly?

Because it's intermittent, I don't know at what point the RCBO is tripping, it could be;

1. when smart switch A turns on amplifier A
2. when smart switch B turns on amplifier B
3. when I start to turn on individual zones, after the amps are powered up.

I have automated (3), incidentally this is done by using a serial connection to my server.

I have introduced timing, i.e. the amplifiers and zones are not all switched on at the same time - 200ms between each switch event. But the problem still happens. I also tried 1s delays.

I understand this is likely caused by inrush current (not voltage spike), and so a surge protection device will not resolve the problem.

Question: What steps can I take to prevent this happening?

Answer 1

I understand this is likely caused by inrush current (not voltage spike), and so a surge protection device will not resolve the problem.

An RCBO may have up to 4 protection methods in it.

• overload of circuit (power above rating for too long, taking pains to allow short term overloads for motor startup, inrush, etc.)
• line-neutral hard/bolted short-circuit exceeding many times the rating of the device
• line-earth hard/bolted short-circuit ditto ditto
• "ground fault" aka differential current, where the hot wire moves 3.674 amps. But the neutral wire only returns 3.633 amps. Therefore 41 milliamps is returning via some other pathway and this is of concern because it could be a limited flow live-earth or neutral-earth arc fault, potentially starting a fire, or shocking a human.

So you can start by interrogating the RCBO for what type of failure it is flagging, if it has the capability to tell you.

Otherwise you can get a plain circuit breaker ( first three functions) in series with an RCD (final function only) and see which one is tripping.

I hate to break this to you, but most leakage faults which trip RCDs are the real McCoy, and people have a strong tendency toward denial that their equipment is faulty.

Answer 2

You either lower the inrush by changing your gear, or get breakers with better current rating. There are 8 A and 10 A models, but an electrician must be consulted to check if wiring works with it, and installation.

Answer 3

The problem may be caused by either (a) inrush current, or (b) earth leakage current, or a combination of the two during the instant of turn-on. To complicate matters, a contributing factor could be the type of "smart-switch" you are using to remotely control the amplifiers. For example, the type of output on the smart-switches may be influencing your situation; typically "smart-switches" are available with at least two (2) different types of outputs, namely:

1. Relay contact, or
2. Semiconductor (SRC, TRIAC, or transistor).

Each of these outputs behaves differently, and this behaviour could be influencing your problem. For example:

• Relay contacts may have "contact bounce", whereas semiconductor outputs do not;
• The on-off timing at the output may either be synchronised or randomised to zero-crossing of the AC supply.

Furthermore, depending on the quality of the smart-switch, these outputs may or may not have "snubbers"; these are circuits placed in series or in parallel with the output switch which are intended to prevent spurious undesirable voltage and current from reaching the load.

So I hope you see that smart-switches have many different types of output, and these will all have varying effects on a connected load (your amplifier, in this case).

Suggested Course of Action
I would suggest the first step to diagnosing the problem would be to look at the AC protective device for the outlet used to power the amplifier (you mentioned it was an RCBO) and determine what caused the trip (some RCBOs can differentiate between an earth leakage trip, or an over-current trip).

If the trip was due to earth leakage then the simple solution may be to power the amplifier from a non-RCBO protected outlet, or to change the RCBO to trip at a higher leakage current (these are usually available in earth leakage trip thresholds of 10mA, 30mA, or 100mA). If the trip was due to over-current, then it may be possible to change the breaker trip-curve setting to tolerant higher inrush current (unlikely for domestic breakers), or consider swapping out the circuit breaker to one that can tolerate a higher in-rush current, for example, a "D-curve" breaker. However, for all of these remedies you will need to check with your electrician to see if the remedy is (a) physically possible to implement (there may be limitations regarding available components, and what changes can be accommodated in your electrical distribution panel) and (b) within the wiring regulations applicable to your jurisdiction.

Another investigation to consider is to run some tests without the smart-switch. Simply connect the amplifier output to a manually switchable AC outlet fed from the same RCBO as it is now, but with the smart-switch removed. Does the problem remain when the AC outlet is switched? If the problem disappears, then the smart-switch may be the main culprit here - but that is not to say it is the only culprit - it could be that it is a combination of factors that causes the unwanted tripping.

If none of the above remedies are available to you or are not effective, then here are the options to eliminate this problem:-
(a) install an external filter between the AC supply and the amplifier AC inlet port, or
(b) modify the amplifier unit.

Option (a) needs more information in order to select an appropriate filter product. I would suggest contacting the manufacturer of the amplifier, explaining the problem and asking them for advice; they may be able to offer guidance as to option (a).

Of course, option (b) should only be a last resort and only done with the agreement of the supplier of the equipment.