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In an isolation transformer (I assume a 220v-110v step down transformer is also called an isolation transformer), you can ground either of the secondary leads. I'd like to know the following:

  1. If you ground one lead, and short the other lead to that ground (Or connect both leads to ground).. would this trip the breaker or since it is an isolation transformer and the current is limited by the core size.. then it can't trip the breaker?

  2. If you touch any lead of the isolation (step down) transformer, you are not supposed to get electrocuted.. but if you touch both leads.. would you be electrocuted or still not?

Biney
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2 Answers2

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If you ground one lead, and short the other lead to that ground (Or connect both leads to ground).. would this trip the breaker or since it is an isolation transformer and the current is limited by the core size.. then it can't trip the breaker?

The current is not limited by the core size, but rather by the resistance of the wires (at a given voltage level). If you short the secondary, the current will increase in both secondary and primary, which will cause the transformer to heat up and, likely, to burn up. If it is a high power transformer, the current in the primary could become high enough to trip the circuit breaker.

If you touch any lead of the isolation (step down) transformer, you are not supposed to get electrocuted.. but if you touch both leads.. would you be electrocuted or still not?

If the secondary voltage is high, say, more than $50$VAC, it is dangerous to touch even one lead, because the second lead could be grounded, in which case the first lead would be at 50VAC relative to ground. If you touch that lead with one hand and touch a grounded object with the other hand, a dangerous current could flow through your body.

If the secondary voltage is low, say, $12$V, you can touch one lead or both leads with no consequences, since the worst case current through your body will be very low. Still, I would never deliberately do it, because a mistake or a broken transformer could have lethal consequences.

V.F.
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  • Anyway. For any step down transformers, is it true that one can connect the neutral/ground to any of the leads of the secondary? I mean.. is there no exceptions or no transformers where this is not allowed? And how do you test whether a transformer can have any of the secondary tapped to ground (or neutral) and no problem? This is because if you touch the neutral to any of the wire in the primary, it would short and spark.. but doing that to the secondary is something new I heard.. so just want clarifications on it safety. Note this question is separate from above where you ground both leads. – Biney Oct 14 '18 at 22:45
  • @Samzun The term neutral is used for one of the live wires. The other two wires connected to an outlet are hot and ground. The two live wires, hot and neutral, are connected to the primary. Although the neutral is connected to the ground at the service panel, it is not interchangable with the ground wire, because it carries the same AC current as the hot wire, while the ground wire carries a current only when the hot wire touches a grounded case (fault current). You should not connect neutral to ground outside the service panel because it will break the safety system. – V.F. Oct 14 '18 at 23:38
  • @Samzun With that understood, the secondary, generally, can be connected to ground, but in many cases it is not needed, not desirable or should not be done - all depending on the application. – V.F. Oct 14 '18 at 23:39
  • The secondary of the step-down transformer needs to be connected to ground to suppress common-mode surge and to avoid capacitive coupling of the primary and secondary. I thought this was always done. Any reference about what application calls for this to be done? – Biney Oct 15 '18 at 00:24
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    @Samzun To start with there is a whole class of AC powered devices, with power transformers, that do not have metal chassis, so there is no way to ground the secondary. Lots of stand-alone power supplies have floating output. Even when the low voltage side is grounded, it is often done after a rectifier - not directly at the secondary winding. As you can see, grounding of the secondary is not a law of nature. – V.F. Oct 15 '18 at 01:13
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  1. You are correct in assuming that saturation of the core will limit the current that can be drawn from the secondary. Whether or not that will trip the circuit breaker depends on the size of the core, and the trip current of the circuit breaker.

  2. If you touch both leads of the secondary you will experience the full voltage of the secondary. If that is 110V or 220V then yes, you will be electrocuted; if it's 5V then you'll be fine.

Remember that it is not the voltage that kills you, it's the current. Hence a low voltage might be lethal if the resistance of the body-circuit is low and the source can provide a high current. On the other hand, if the current is limited, touching thousands of volts may be unpleasant but won't kill you. Anything over 100mA can easily kill you.

hdhondt
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  • If the trip current of the circuit breaker is say 20 ampere, what must be the size of the core? If it is too small, you mean it won't trip the breaker even if the secondary leads are already short circuited? – Biney Oct 14 '18 at 00:17
  • There are other factors that determine core size as well, e.g. core metal type, number of winding turns, and cooling. Have a look at http://engineerexperiences.com/design-calculations.html . And yes, if the saturation current is too low it won't trip the circuit breaker. – hdhondt Oct 14 '18 at 22:16
  • Core saturation does not limit the current that can be drawn from the secondary. A transformer will limit the current that can be drawn from the secondary, but it is not because the core saturates, but because there are impedances (resistances and reactances) that limit the flow of current. A transformer designed to operate at a given primary voltage and frequency without saturation will not saturate if the rated voltage and frequency are applied even if the secondary is shorted. It may burn up, but the core will not saturate. – Math Keeps Me Busy Feb 04 '22 at 21:48