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In these 2 output-wires, which-one will act as live, and which one neutral

  1. In a single-phase transformer input, one wire getting entry as live, another as neutral. But what would be in case of its output? In output, which-wire would be phase, and which-one would be neutral?

  2. Is it a possibility that both the output would be "live" (of opposite type) (according to the drawing)? or one will act as phase and the other neutral?

  3. The stepdown transformers I buy from market, for small gadgets, (are not like those diagrams (as I drawn)); contain a center-tap. I guess, the center tap is used to resemble the neutral wire. is it so? or there is some-other cause behind using that center tap? does same mechanism (center taps) used in high-voltage transformers also?

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5 Answers 5

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  1. It's very simple. Neither is neutral until it is "neutralised" by connecting it to ground. Until this is done the outputs are floating or isolated with respect to ground.

  2. Yes, polarities will be opposite.

  3. Again, until the centre-tap is grounded all three terminals are floating. When grounding one could ground either the centre tap or one of the outer taps to get various voltages. e.g., a 110 - 0 - 110 transformer might normally be centre-tap grounded to give two 110 V outputs 180° out of phase but could be grounded at one of the outer terminals to give a 0 - 110 - 220 V output.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Floating output. Figure 2. Neutralised output. Figure 3. Centre-tap grounded. Figure 4. Outer tap grounded.

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  • \$\begingroup\$ if i pull the lower line to the ground , how does it raise the voltage on the upper line ? \$\endgroup\$
    – Dogus Ural
    Commented May 11, 2016 at 7:31
  • \$\begingroup\$ The transformer has 110 V between centre-tap and each outer terminal. It has 240 V between terminals. Standard practice is, on earth referenced systems, to refer to voltage with respect to ground. In figure 4 we have grounded the bottom terminal so the centre is now 110 V away from ground. Since the top terminal is 110 V away from the centre tap it must now be 220 V away from ground. You can have the same thing with little 9 V batteries. Connect two in series and, depending on which terminal you connect to ground you can have 0 / 9 / 18, -9 / 0 / +9 or -18 / -9 / 0 V arrangement wrt ground. \$\endgroup\$
    – Transistor
    Commented May 11, 2016 at 9:09
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Neutral (at least in the USA) is connected to ground (at one point, the first panel in the building). Therefore, as shown, neither output wire is neutral. It is closer to your option 2, both live, though both are isolated from ground.

If you don't need isolation, the proper way to use this would be to connect one wire to earth and consider that one neutral.

As far a center tapped transformer, again everything is isolated from ground so phase and neutral don't really apply. Again you could earth the center tap to make it equivalent to US split-leg 240V where both legs are hot.

As for high-to-consumer voltage transformers, outputting 120/240 volts, then normally a CT transformer is used to produce the split-leg 120/240V. The input is typically two phase wires from a 3-phase distribution.

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  • \$\begingroup\$ Aropund here, the input to the HV step-down transformer is normally between one HV phase and Ground, not between phases. \$\endgroup\$ Commented May 11, 2016 at 6:30
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A new take on an old topic... Euro-spec, mind you.

When 230VAC line voltage is connected to a 230/24VAC or any 2-pole transformer it will definitely have a hot and a cold leg. I don't recommend touching either one with bare hands, though.

The secondary negative leg is symmetrically on the same end of the coil as the line neutral just like the hot side is on the other end. Just because AC oscillates doesn't mean the charge isn't on the line side (where the spinning 3-phase generator has propagated an imbalance of electrons) while the neutral wire is there only to complete the circuit ending in the main panel's neutral bus that's connected to the ground bus (and thus the actual soil) by a heavy slab of metal.

Another common misconception seems to be that you can never get shocked by the neutral wire. That of course doesn't apply if the neutral wire is not connected in the panel/ground end while the hot wire is. That's why you always disconnect the line before the neutral so a situation where a loose neutral wire would have a voltage on it can't occur. In such a case it is just an extension of the hot wire looking for the shortest route to ground which could be you.

The 18W 230/24VAC iron core transformer I use to power 3-10W attenuators at work usually has 2-3VAC to ground on the negative or "neutral" side while the positive has the transformers output voltage minus the (common mode) voltage in the negative pole. The system is thus floating as it has no direct reference to the general ground potential of it's surroundings and the potential between + and - is above the transformers nominal voltage at 26-28VAC corresponding to 228-237VAC mains. It is, however, not very far off from the common ground.

While a floating system has it's advantages and most household appliances are safety isolated that use a two-pin plug which has no contact to ground problems arise when voltages and signals from different sources and potentials meet at some point. Digital signals and voltage controls often rely on a common reference ground. If a devices power source has a significant difference in ground potential to the signal source's ground potential errors and deviations start to occur.

To bring everything neatly to the same ground potential a transformer's secondary leg can be connected to the same neutral bus as the negative sides the other voltage and signal sources. Although it's often advertized that either leg can be grounded and the other becomes the positive side I don't see any practical reason to ground anything than the less positive side corresponding to the mains' neutral side.

The system won't be isolated anymore but will be more in tune. It also adds a safety measure because the grounded leg provides a low-Z path to ground when a transformer is turned off or breaks down sending a high current/voltage spike as the magnetic field suddenly collapses.

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I use a no-contact AC tester (12-600 VAC), even though the secondary is floating with no ground. There is a Hot lead and there is a Neutral lead.

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    \$\begingroup\$ If the secondary is floating with no ground, there is no Hot lead and there is no Neutral lead. \$\endgroup\$
    – Uwe
    Commented Sep 19, 2022 at 8:50
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Both acts as both line and neutral because transformer is an AC device so we cannot predict which side does the current flows.

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    \$\begingroup\$ Welcome to EE.SE. "Both acts as both line and neutral ... No, neither is neutral until it is neutralised. (See my answer.) "... because transformer is an AC device so we cannot predict which side does the current flows." Equal current will flow in each line. We can predict the current flow. \$\endgroup\$
    – Transistor
    Commented Feb 9, 2019 at 11:08

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