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Harper taught me how to connect the following and it's working perfectly. From (related to) the post Connecting a Pseudo-Neutral to Pig Tail of GFCI where I only have 240v black and red line of the US ac power in the Philippines and the Siemens GFCI has 120v circuitry. So Harper suggested an autotransformer with neutral to power it and worked flawlessly.

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Harper, etc. Can I add the following to the above?

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I mean the Neutral and Ground will be connected to the autotransformer centertapped output with the 2 Hot connected to the autotransformer input? This is to avoid surges damaging the sensitive GFCI circuitry (the internal MOV may not be enough).

Note I can easily use the Siemens First Surge by simply shorting the ground/neutral leads and not connecting it to anything and just connecting the two hots. No alarm sounded and the Siemens First Surge works without error.

But this won't offer the L-N protection mode with lower VPR (600) instead of L-L of 900v VPR (related to Let-through voltage). Hence I need to use 120v protection mode too by connecting the neutral/ground of the Siemens First Surge to the autotransformer neutral lead (which powers the Siemens GFCI breaker circuitry).

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Remember when I warned you not to get tempted to use the pseudo-neutral for anything else? This.

This is exactly the sort of "scope creep" I'm talking about. "Oh, I've got another thing that needs neutral, I'll just grab it from there". Nuh-uh. No way.

See, the GFCI uses neutral only for its own internal power, it's a tiny load. The surge suppressor doesn't use neutral for internal power, it uses it to suppress surges on the neutral: it aims to flow hundreds of amps down the neutral.

That would vaporize the tiny transformer.

It's also possible the surge suppressor has some lights on it. Those are probably to indicate faults such as a lost pole or lost neutral. The latter being particularly dangerous in North America, but a big nothingburger in your part of the Philippines.

(To our readers, the Philippines is in the midst if a transition between North American 120/240V and European 230V. The first step of the process was deleting center neutral. Those neighborhoods get 2 legs of power, neither one near ground. This is unlike Europe, where one of the legs of power is neutral and thus near ground.)

The simple fact is, surge suppression in the Philippines is a difficult problem. At first glance, Euro style surge suppressors are a better fit, because they only protect surges on 2 poles. However, Euro units expect there to be a neutral that is near ground, so they may not br a good application.

Indigenous units actually built for the Philippines would be the best choice. Otherwise you have two options, and I would consult with the factory:

  • hook the surge suppressor's neutral and ground to your earthing system.
  • if the suppressor can stand 240V across hot-neutral, wire both hots to one leg of power and neutral to the other leg of power. But I would not do that without factory consultation.

Keep in mind, you are seeing the spec call out a higher voltage across the 240V wires because it should be that way for 240V. You can't outsmart the factory, they are pretty smart.

  • I had another Siemens First Surge SPD I had been using for 5 months already. I know it uses 240 to power circuit. It has 150v MOV across the L-N. When connected L-L. It uses the two 150v in series. Remember our power system is centertapped so it measures 120v to soil.This is why the Siemens SPD is compatible. About it vaporizing the tiny transformer. U mean f it is connected L-N to protect 120v (that is protecting surges getting into the Siemens GFCI breakers),the surges has no actual neutral/ground to go so will stay in circulation and can destroy the 100va autotransformer powering them? How? – Samzun Jan 25 at 19:10
  • To add to the above comment. MOV are sacrificial elements. When put line to line. It doesnt make current flow but the SPD acts as a voltage divider with that supply impedance to reduce the peak voltage at the terminals of the SPD. Similar when put line to neutral. Why would this destroy the autotransformer when connected to its centertap? Won't it only protect it? – Samzun Jan 26 at 1:01
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    Because surge suppressors don't "magically make surges disappear" they have to send the surges somewhere. This one is designed on the assumption that the neutral is a big fat 2/0 aluminum wire going up to a pole transformer, and it won't do any harm to throw 500 amps up that wire for a few milliseconds. Yes, that is an edge condition. But surges are edge condtions. The way you protect that tiny transformer from surges is by protecting the two wires supplying it. There's no useful use to protecting that tiny pseudo-neutral, since that doesn't go anywhere but these GFCIs. – Harper - Reinstate Monica Jan 26 at 1:20
  • But do you agree that lower voltage has lower let thru voltage? For example..in tests the VPR (voltage protection rating) of 150v MOV is about 500v vs the VPR of 300v MOV which is about 1100 volts. This is when tested with 6000v. 3000a. 8/20 usec surge pulse. Therefore 150v MOV is more protective bec of lower let thru voltage appearing at the load, right? Hence 150v mov in the siemens gfci breaker 120v circuitry is more protective to its sensitive circuit components. – Samzun Jan 26 at 2:26
  • my suggestion to connect both hots to one hot, and the neutral to the other hot, would have the effects you want. However as said I would not do that without factory advice, so what I am saying is "ask Siemens". – Harper - Reinstate Monica Jan 26 at 3:40

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