Isolation transformer

[wkyong]

As an example : A 1:1 ratio and1 primary 1 secondary isolation transformer

1)why we need to bond secondary neutral to earth?what is the purpose of it?

2)if we are not doing so what would happen?will it generate noise?

3)if human body accidently touching any secondary windings of isolation transformer ,would it trip off the Residual current circuit breaker(RCCB) placed at the input primary?

[Justmanuals]

Yea? So what?

[minnie]

[torbjorn]

1) The purpose of this is that a RCCB connected on the secondary side of the insulation transformer shall have a possibility to trip, and to ensure that a primary RCCB or fuse will trip in case of a fault within the transformer. Whether a such bonding is required, optional or prohibited depends on the local electrical codes and the exact application of the transformer, so it is impossible to answer without having more facts in the case. The best is to study the codes and ordinances that are valid in your area.

2) If the transformer has a high degree of capacitive coupling between primary and secondary (both windings concentrically on the same bobbin and no grounded screen between them), then high-frequency noise can be coupled from the mains to the secondary winding. If the transformer is of a such design and you have to avoid grounding the secondary, then make sure that the end of the primary winding that is nearest the secondary, is connected to the mains neutral.

3) No. If you want a good personal protection (for instance, if the isolation transformer is to be used at a workbench where power supplies are troubleshooted and repaired), then the secondary should not be directly grounded. Perhaps, the mid point of the secondary could be grounded via a high-value resistor (1 Mohm or so) to avoid static charging.

[PackratKing]

OK maybe the Packrat is shooting himself in the foot.

I have several really nice identical isol. trans., one feature of which, puzzled me.
Out the bottom is a green wire, from between pri / sec, with no continuity to any other lead or the laminations.

Is there something in there burnt out ? or is it as torbjorn suggested, a grounded plate to defeat capacitive coupling. -- I'd hate to have to relegate them to copper scrap !

Windings: 5 equal voltage taps off the secondary, and a honkin' high-amp winding, about 50 turns of #10 ! voltage as yet unk.

Lamination stack is roughly 5 x 6 " x 2" thick, whole unit approx 20 lbs.

[vtech]

[PackratKing]

I was bordering on bummed out, hoping there was nothing wrong with my monsters.. I will definitely make sure this lead is used properly when designing any projects.

In wiring projects, is it "legal" to feed the primary of another downstream transf. off the output tap of the big one ??
I Imagine a common chassis ground, or should I go back to that hot leads' common on the main trans.

Methinx it's time to get out the old textbook..........

{{ I gotta get cooking on including drawings or pix in these posts }}

[vtech]

[PackratKing]

I guess my terminology left a little to be desired.

I am concerned with cascading transformers, where the successive primaries should be grounded, back to a common chassis ground, , or back to the first trans. in the string...?.......

[torbjorn]

I don't see that it ever would be of much use to cascade several isolation transformers. It would just get problems with voltage drop, high inrush currents, unnecessary idle losses and so on.

Whether it is legal or not, i must say that i don't know much about the electrical codes in the US, more than that the legislation may vary a lot from state to state.