transformer and main supply.

A transformer output is typically electrically isolated from the supply. It may thus possible to do things to it, such as short either terminal to ground, touch either wire without risk of serious shock, superimpose a large DC (or AC) voltage on either terminal wrt neutral or ground, etc. that are not possible with the power source itself. Depending on the transformer, its output may be impedance limited, so it is possible to short the output wires together without causing excessive currents to flow. This also means that a transformer output will be less well regulated than the supply and the voltage may be significantly higher than nominal on little load.

The technical difference as Palindrome points out is correct, however I assume it's the criteria applied that's causing the confusion.

L1, L2, L3... etc are accepted standards of what to call electrical connections on certain types of electrical equipment.

L1, L2...etc are used on transformers. Connect cables to them and run the cable into a buildings consumer unit (Fuse box etc) they become L & N. Exactly the same connections but renamed to suit where the connections are and to tell you what they are intended for.

In motors and generators you'll find things like u1, u2, u3, U1, U2, U3.

Newsey

Besides this, an "isolation transformer" may feed a full-wave bridge rectifier, and one terminal of the output DC can then be Earthed. There are also other configurations that give both rectification and voltage multiplication.

This used to be the "norm" with valve (tube) power supplies. Today it is still done for the larger transmitters (ex 500W upwards), with voltage multipliers providing 1000v+ DC for higher powers.

All require a transformer with an isolated secondary.

Most electronic workstations require a "mains isolation transformer" as a legal requirement. There is still equipment around with "live chassis" (thankfully not much), and an equipment fault with a power-line short to ground could otherwise cause the equipment to become dangerous to the person employed to find the fault.

transfromer

The former is a single phase supply; with one line (L), of the three phases and a Neutral from the star point of a transformer. This is a standard supply, say for the UK at 50 Hz. The other is a two phases from a three phase supply (L1 and L2), the other phase is L3.

Jaymack

A little anecdote to this thread:

When I was an apprentice in engineering college (UK Military, in the 60's), I had an instructor who's job it was to teach us "basic electricity". Back in the barrack block in the evenings I was building my second radio transmitter. It delivered 40-Watts, using an 807 final stage.

During one of the first daily lessons the instructor, a Flight Lieutenant Barker, told us:

"Current cannot flow unless there is a complete closed circuit. If I had a source of voltage: two contacts with 1000 Volts between them, then I could safely touch just ONE contact and not feel anything at all. I could only get a shock if there current flowing from one terminal to another, or Earth."

The following day we all sat in the classroom, and the following conversation took place:

FS: "Lythall! What IS that hideous contraption on your desk?" ME: "Yesterday you said that if you had 2 terminals with a 1000 Volts ..." FS: "Yes, but what is that to do with that - er - THING on your desk?" ME: "This thing, sir, is a 350v-0-350v mains isolation transformer and there are 700 volts between these two terminals, here. Yesterday you said you could touch them and not feel a thing."

Old "Barky" (although he was quite young) was quite brave and took up the challenge. He used a 1000v Bridge MEGGA to check the primary/secondary isolation, then he wired 3x mains light-bulbs in series and connected them to the secondary. He stood on dry wooden duct-boards, and a rubber mat, and then plugged in the transformer. He measured the voltages with an AVO for us all to see. He put one hand in his pocket and touched one terminal with his other hand. Nothing! He was most definitely alive. He then moved his hand to touch the other terminal, and with a painless grin on his face, yet these three 240v lamps were still grinding red-hot.

FS: "Now Lythall, do you believe that I can safely touch one terminal?" ME: "Yes sir!" FS: "Do you have confidence in my teachings?" ME: "Yes sir!" (Then with an evil glint in his eye he said:) FS: "Good! Now Lythall, it is YOUR turn to come up here and show the rest of the class just how safe it is. You can repeat what I have just done."

Those were the days! There is no way a teacher could ever use such teaching techniques today. But we all learned a lesson that day. Although I was quite advance, far ahead of the other students, thanks to Amateur Radio, I still learned to treat instructors with respect. I also gained a lot of self confidence, for a mid-teenager.

Best regards Hairy Lethall - oeieio(at)hotmail(dot)com

That I can understand. As I said, things were a little bit different in the

60's ;-) Even the transmitter I was building was for the "Echo Charlie" pirate AM band (6.5 - 6.7MHz), and that got people wincing a bit in the Hamradio NGs ;-)

In another newsgroup, an electrical power engineer confirmed that when he signs off HV switchgear, equipment, transmission lines, etc as being safe to work on, he is expected to reach inside a cabinet and physically touch a normally live conductor in front of the crew who are going to do the work. He said that this is standard practice, and that it concentrates the mind wonderfully when deciding which isolators to open, and which lines to put earthing links on.