The GTL-POWER VA15-V2S12 part you are using is not a transformer. It is a switch mode power supply ac to dc converter module. The FG pin might be "Filter Ground" but it's use is not mentioned in the datasheet and from the "EMC" applications drawings, it does not appear to have any internal input side filtering. I personally would not touch it with a barge pole without implementing their example external filtering.  You can try asking the manufacturer: [http://www.gtl-power.com/](http://www.gtl-power.com/) They have a chat popup so try "The GTL-POWER VA15-V2S12 has a pin named FG. It is not shown in the applications diagrams. What is the purpose of the FG pin on the GTL-POWER VA15-V2S12?"

L = live N = neutral FG = Earth / Ground (yellow/green wire in EU countries) [https://en.wikipedia.org/wiki/Earthing_system](https://en.wikipedia.org/wiki/Earthing_system) [https://en.wikipedia.org/wiki/Ground_(electricity)](https://en.wikipedia.org/wiki/Ground_%28electricity%29)

@markus_jidoka, "FG = Earth / Ground (yellow/green wire in EU countries)" That is an assumption on your part and is not supported by the device datasheet. Your references are informative but shed no light in the particular device because there is nothing in the device datasheet that suggests the usage of the FG pin. Connecting the FG pin to a ground somewhere may be OK or it may not. Since the datasheet shows it unconnected in all the applications drawings, the safest thing to do may be to leave it unconnected. In cases of such poorly documented devices however, particularly where electrical safety and standards compliance are issues of concern, I would advise choosing a different, better documented device. For example: [https://uk.farnell.com/mean-well/irm-15-12/power-supply-ac-dc-12v-1-25a/dp/2815499](https://uk.farnell.com/mean-well/irm-15-12/power-supply-ac-dc-12v-1-25a/dp/2815499) [http://www.farnell.com/datasheets/2547886.pdf](http://www.farnell.com/datasheets/2547886.pdf) which includes the EMI filtering and is unambiguous about the mains side connections.

@GerritMax, @markus_jidoka, I contacted GTL Power and from the response below it is clear that, on the VA15-V2S12, the FG pin is an unconnected pin. "The FG pin is an empty pin inside the power supply, which is mainly used for welding and fixing the product, and the client does not need to be connected with any electrical connection. At the same time, it is also a reserved ground pin. If some customers need to use it for grounding, our power supply will be changed internally to become a ground pin, **but this special model design**." "FG引脚在电源内部是空脚，主要起到产品焊接固定做用，客户端使用不用与任何电气连接。同时也是预留的接地引脚，如果有些客户需要用来接地，我们电源内部会更改，变为接地引脚，但这种这特殊型号设计"