220 to 110 transformer ground issue?  - Page 1
Forum section: electronics / beginners

author: paulhm81
author_rank: Regular Contributor
timestamp: February 16, 2017, 07:18:47 pm
content: HelloPlease help me understand this scenario!A friend of mine brought home (romania) some equipment from the us and among them a tv. He also brought a transformer to power it because it only shows 110 on the input. He asked me to look at the transformer because it started breaking the differential fuse every time he plugged it in. I opened it up before plugging it in to see what's happening and I discovered it's an autotransformer. There is a sticker on the back that says if the load has a ground pin to check the correct polarity (in my country you can input the plug either way). I powered it and it is working fine and that is what I don't understand. Is the neutral connected to the ground in us equipment?Thank you!

author: helius
author_rank: Super Contributor
timestamp: February 16, 2017, 07:29:27 pm
content: An autotransformer doesn't isolate the input and output.So one side of the input is connected straight through to the output, at the same voltage. If the input plug is not polarized, this means 220V could be present at the output.

author: paulhm81
author_rank: Regular Contributor
timestamp: February 16, 2017, 07:41:19 pm
content: Thank you!To check if it is properly connected I sould check with a dmm between ground and the outputs so I don't get 220, right?But why is the differential breaker triggered?

author: Gregg
author_rank: Super Contributor
timestamp: February 17, 2017, 01:12:49 am
content: The differential ground breaker is being opened because it sees an imbalance in the two 220V leads.It would help to know how the grounding conductor is referenced to the mains in Romania. I tried a quick google search but mostly found a bunch of stuff on the type of plugs used and some information on high voltage electrical distribution.You may have to get a true isolation 220V to 115V transformer instead of the autotransformer. Then the output will be balanced and totally isolated form the mains and one leg could be tied to the earth ground and labelled “neutral” to work the same as the US power system. The neutral is the larger prong for two prong plugs and usually colored white. It wouldn’t hurt to not ground one of the 115V isolation transformer leads if your equipment uses a three prong plug where you would connect the earth ground to the larger rounded prong, or if you can connect the earth ground directly to the equipment chassis.

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 03:34:16 am
content: It is my understanding that there should never be any direct connection between mains Neutral and Ground inside any piece of equipment that gets plugged into a wall socket ... that the ground is carried through, isolated from the mains circuitry and connected to any exposed metallic parts. It may be connected to the operational circuitry as a reference point or for interference or electrostatic reasons - but there will be an isolation point somewhere between the mains and the ground.As I understand it, it should not matter which way the autotransformer is connected, nor the load - UNLESS - there is a leakage path somewhere, which is going to trip the breaker. This is what I would be looking for to start with. (It may be that any such leakage path is sensitive to the Active/Neutral orientation.)To aid in any investigation into this problem - does the breaker trip withonlythe autotransformer connected -or- when the autotransformerandthe load are connected?

author: Gregg
author_rank: Super Contributor
timestamp: February 17, 2017, 06:23:28 am
content: Quote from: Brumby on February 17, 2017, 03:34:16 am

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 06:52:59 am
content: There have been several discussions on the domestic power arrangement found in many US households - but this topic is not about that.We are talking about a 110v TV that has been taken to Romania, which runs on 230v.

author: paulhm81
author_rank: Regular Contributor
timestamp: February 17, 2017, 07:03:11 am
content: The ground is connected to earth at some point in the delivery path. In my case the circuit breaks only with the load and only if the transformer input plug is connected the "wrong" way. To my understanding if the load does not have an earth plug it works but if the load has one you must check that the transformer is correctly plugged in. If I'm not wrong, the differential measures the difference between what enters and what exits and if there is a difference it triggers so there has to be a leak between ground and earth in the tv. Could it be some decoupling capacitors?Anyway, the problem is solved but I'm just trying to understand the issue!

author: mtdoc
author_rank: Super Contributor
timestamp: February 17, 2017, 07:08:44 am
content: Quote from: Gregg on February 17, 2017, 06:23:28 amYes, it's ridiculous what it costs to get a copy of the NEC.FWIW if you go to the National Fire Protection Agency website you can sign up and get a 2 day free subscription to the NEC online. I've done this a few times with different emails when needed. They have an embedded webviewer app so no easy way to download it unfortunately.

author: paulhm81
author_rank: Regular Contributor
timestamp: February 17, 2017, 07:19:00 am
content: This is interesting!http://www.gson.org/stepdown/

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 07:26:40 am
content: Quote from: paulhm81 on February 17, 2017, 07:03:11 amMy guess is that this will be when the common terminal of the autotransformer is connected to the Active side of the mains.QuoteYou are not wrong.QuoteYes - if the TV has an earth pin ... or if there is an electrical connection to another piece of equipment which is grounded (eg connection to external sound system.)QuoteIf you are thinking of mains input X/Y class capacitors, then this is a possibility - IF the TV has a plug with a ground pin.

author: Brumby
author_rank: Unknown
timestamp: February 17, 2017, 07:29:26 am
content: Quote from: paulhm81 on February 17, 2017, 07:19:00 amI had not seen that. It is not long , well worth a read and fits in with the issue you have presented.

author: madires
author_rank: Super Contributor
timestamp: February 17, 2017, 09:13:40 am
content: Quote from: paulhm81 on February 17, 2017, 07:19:00 amAnother issue is that some US devices with a polarized power plug and no earth pin got a direct connection between N and the chassis. If you create a current path between the device's chassis and earth the RCD might be triggered. The proper solution is to add an isolation transformer and to connect the output's N (polarized socket!) to earth. It could be a transformer 230V to 110V (prefered), or a 230V isolation transformer plus the 230V to 110V autotransformer. No worries about the Schuko plug anymore.

author: R005T3r
author_rank: Frequent Contributor
timestamp: February 18, 2017, 09:47:22 am
content: Quote from: Brumby on February 17, 2017, 03:34:16 amThe problem with autotransformers are that if by accident the coil breaks after the secondary lead, you will have your appliance working at 220V.If you connect the autotransformer in the opposite way, you will get 220*2 = 440VSpeaking about the breaker, I don't know, it happend also to me a while ago with a brand new variac... I still have to figure it out yet. Maybe, this could be related to the first core magnetization...

author: Assafl
author_rank: Frequent Contributor
timestamp: February 18, 2017, 11:09:47 am
content: Be VERY careful. I had nearly 1000$ worth of damage because of a Chinese auto transformer.What happens is that the secondary may be between N (~0v) and 110v or between 110v and 220v. In theory, both cases are identical. But in practice there is more leakage between 220v to ground, etc.But there is a far Worse scenario. If there is a surge suppressor (inside the device, or in an extension cord or splitter) it will probably have 140-150 MOVs wired between the poles and ground. If the voltage is between N to 110v - no problem, if however the 110v is between the phase and 110v, you will have 220v on the Varistor, which will short.At that point (for me) the ground connection turned out to be poor and it lifted the ground high, which fried a very expensive HDMI board from MDS.Since the I test (for correct connections) all transformers that get used around the house - many needed re-wiring.