US Surge Protector trips RCD

[NorthernLights]

Hi everyone, I need some advice please.

I have a number of 110v equipment from the US that I need to supply power to.

When using a step down 240 to 110 transformer when I plug in the 110v power Strip with built in Surge protection it trips the RCD on my consumer unit.

I read up on this a little and it seems this is a potentially dangerous situation.

Can someone explain how I can solve this other than having to buy all new 240v equipment?

Best wishes,

Nathan

 

[pc1966]

That sounds odd, most likely is you are using an auto-transformer (or even some dubious electronic replacement) and not an isolating transformer (they cost more due to the extra copper needed) and somehow the power strip has enough leakage to earth to trip the RCD.

What sort of current do you need for your 110V equipment? Spending something like £70 or so on a decent site transformer might be a better option:

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[Lucien Nunes]

If your transformer is an autotransformer, a possible cause is that the common terminal is connected to the line conductor of the 230V input rather than neutral. Although the voltage between the output line and neutral is correct, the voltages to earth would be wrong, at 230V and 230-120=110V. The conductor at 230V from earth then triggers the surge protection which attempts to clamp what it sees as an overvoltage to earth and trips the RCD. This setup is also dangerous due to the presence of 230V on parts only insulated for 120.

Do you have a multimeter rated at least Cat II and the experience to use it safely on live mains voltage equipment?

 

[dmxtothemax]

Most likely your surge protected power strip has a path to ground,
not unusuall as most power filters use capacitors to ground as part of the filter.
RCD's dont like capacitors to ground.

 

[NorthernLights]

Thanks for the replies so far.

I don't have a multimeter to test the setup unfortunately.

If I just got a regular US style power strip without surge protection built in would that work?

 

[pc1966]

If I just got a regular US style power strip without surge protection built in would that work?

It might stop tripping the RCD, but that would be masking the potentially serious underlying reason as to why it is tripping it.

 

[NorthernLights]

I thought the reason it is tripping is because you shouldn't connect an item with built in surge protection to a step down transformer?

 

[pc1966]

If the transformer is delivering the voltage expected of the surge protection then nothing should happen.

But if (as Lucien Nunes pointed out) it is delivering not 0-110V for N-L but 120-230V instead, it might be satisfying the requirement for 110V between your L & N, but they are elevated well above 110V to Earth and the surge protection might try to stop that resulting in the RCD trip.

 

[NorthernLights]

If the transformer is delivering the voltage expected of the surge protection then nothing should happen.

But if (as Lucien Nunes pointed out) it is delivering not 0-110V for N-L but 120-230V instead, it might be satisfying the requirement for 110V between your L & N, but they are elevated well above 110V to Earth and the surge protection might try to stop that resulting in the RCD trip.

Thank you.

So what is my safest and easiest solution to this problem?

 

[Megawatt]

Hi everyone, I need some advice please.

I have a number of 110v equipment from the US that I need to supply power to.

When using a step down 240 to 110 transformer when I plug in the 110v power Strip with built in Surge protection it trips the RCD on my consumer unit.

I read up on this a little and it seems this is a potentially dangerous situation.

Can someone explain how I can solve this other than having to buy all new 240v equipment?

Best wishes,

Nathan

Nathan I have one question did you bond the neutral on the secondary side of the transformer?

 

[NorthernLights]

Nathan I have one question did you bond the neutral on the secondary side of the transformer?

I'm sorry but I don't understand what you mea. By this.

I've attached an image of the transformer I'm using.

 

[Megawatt]

I'm sorry but I don't understand what you mea. By this.

I've attached an image of the transformer I'm using.

When I say bonded I mean tie the neutral to the ground wire but since it is an auto transformer it should already be bonded on the inside when you plug it up. I’m sure your male plug has a ground prong

 

[NorthernLights]

When I say bonded I mean tie the neutral to the ground wire but since it is an auto transformer it should already be bonded on the inside when you plug it up. I’m sure your male plug has a ground prong

Yes it's a normal 3 pin UK plug.

When I plug any of the devices I want to power directly in to the transformer it works absolutely fine. It's only when I then plug in the power strip that the RCD is tripping.

I've tried another identical power strip and the same happens with both, therefore I suspect it is because of the inbuilt surge protection?

 

[pc1966]

That is an autotransformer, as it clearly says on it! But it is a good one to have, so there is nothing wrong with it as such.

The obvious things to check are the wiring of the UK 13A plug to it (making sure the L & N are the right way) if it is not a moulded plug. Also have you got a socket tester to make sure your 13A outlet(s) are wired the right way round? That is rare, but can be very dangerous as it puts the fuse in the neutral line.

 

[Megawatt]

Yes it's a normal 3 pin UK plug.

When I plug any of the devices I want to power directly in to the transformer it works absolutely fine. It's only when I then plug in the power strip that the RCD is tripping.

I've tried another identical power strip and the same happens with both, therefore I suspect it is because of the inbuilt surge protection?

I would have to agree with you. Good luck

 

[NorthernLights]

I would have to agree with you. Good luck

So is just getting a non surge protected power strip going to work and be safe given the fact U have RCD protection on the circuit?

 

[Megawatt]

So is just getting a non surge protected power strip going to work and be safe given the fact U have RCD protection on the circuit?

I would have to say yes since it’s already RCD Protected

 

[NorthernLights]

That is an autotransformer, as it clearly says on it! But it is a good one to have, so there is nothing wrong with it as such.

The obvious things to check are the wiring of the UK 13A plug to it (making sure the L & N are the right way) if it is not a moulded plug. Also have you got a socket tester to make sure your 13A outlet(s) are wired the right way round? That is rare, but can be very dangerous as it puts the fuse in the neutral line.

Ok so it is a moulded plug on the transformer. I've checked the socket using my socket tester, all good.

I think I've discovered the issue. If the power strip is connected to the transformer when I plug it in then it trips the RCD.

However if I plug the transformer in, give it 10 seconds and then plug in the power strip, it then works fine.

Is this to do with inrush current on startup perhaps?

 

[pc1966]

However if I plug the transformer in, give it 10 seconds and then plug in the power strip, it then works fine.

Is this to do with inrush current on startup perhaps?

Good to hear there are no serious wiring issues.

It sounds like on switch on the transformer is kicking out a short high voltage spike that is enough to exceed the power strip's L-E voltage protection, so in a sense it is doing its job, but equally it is not convenient for you!

Good to know you have a work-around for the problem!

 

[pc1966]

If you want a better long-term solution you could get a non-protected US distribution strip and add a UK style surge protector to the autotransformer input.

With the 18th edition of the IET wiring regulations surge protection devices (SPD) at the supply to a home became a factor to consider but I suspect very few home will have that for years to come.

 

[UKMeterman]

Ah, if it is a double pole switch and it closes live first then the whole lot will be at 230V above earth and the surge protector will then provide a path live to earth. Also the transformer will take a while to settle down and could provide some interesting spikes.

 

[Megawatt]

If you want a better long-term solution you could get a non-protected US distribution strip and add a UK style surge protector to the autotransformer input.

With the 18th edition of the IET wiring regulations surge protection devices (SPD) at the supply to a home became a factor to consider but I suspect very few home will have that for years to come.

All new homes built or service changes will have to have SPD’s located at the main panel starting this year according to the 2020 edition of the NEC

 

[pc1966]

All new homes built or service changes will have to have SPD’s located at the main panel starting this year according to the 2020 edition of the NEC

That is good to hear, basically same ethos as our regulations!

While most mains stuff should be OK with 4kV spikes a lot of consumer electronics is crap and so it not

 

[Lucien Nunes]

I like the DP switch explanation, and given your observation that it only trips at switch-on, either that or the inrush transient is almost certainly the cause.

RCD's dont like capacitors to ground.

In the absence of any reference to the size of capacitors, I think that's too broad a generalisation. Many appliances and electronic goods have capacitors to ground and are intended to operate from an RCD. Looking at my bench equipment, I probably have 40 separate devices with Y-caps to ground all running from the one RCD, IIRC the leakage is around 8-10mA. A standard power strip made for the USA market would be intended to run from a 6mA GFCI which may trip from 4mA upwards, so would only be permitted to leak a fraction of that current.

However, you may well have a point that in this case the presence of the capacitors is increasing the leakage pulse due to the transient.