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Safety Isolating v Isolating Transformers

Les Spiteri
Anybody able to explain the difference to me as I hear both terms used?
  • I see centre-tapped maybe the difference with this being a safety isolating.

  • Whereas 0-110V is simply isolating?


And when and where would an application suit either? Normally these days all control panel transformers are 0-110Vac for control although quite often when you debug older panels you will see Centre Tapped ones?
  • 0
    A safety isolating transformer (e.g. SELV) has a separate layer of insulation between the primary and secondary windings, so it's not relying just on the thin lacquer coating on the coil wires for protection.


    A centre tapped transformer, where the centre tap is earthed - like the "yellow" supply at a building site, gives a decent voltage end-to-end (e.g. 110V) while ensuring that any live conductor is, e.g. no more than 55V from E.


    The two techniques are not mutually exclusive.
  • 0
    EN 61558-2-6  (safety isolation) double insulated and fail safe design including thermal fuse.

     EN 61558-2-4  (just isolation) lower test limits. Need not be fail to safe, any old double wound construction.

    Different symbol  there is a shield outline around the two circles.
  • 0
    I think the same applies as I posted on another thread regarding earthing the 0V of a 24Vdc power supply, where a single fault on a centre tapped transformer can leave a voltage where it should not be. Therefore with regards control its always in my experience not correct to use centre tapped. 


    I found this on another thread:


    EN 60204-1 clause 9.4.3 would apply in this case.

    There are 3 methods, one is a conventional "TN-S" supply where the 0V / d.c. - ve is tied to earth.

    The second is an unearthed system with earth fault monitoring.

    The third is a CTE system.

    However, whichever one of these is used the overriding statement is:

    "9.4.3.1 Earth faults

    Earth faults on any control circuit shall not cause unintentional starting, potentially hazardous

    motions, or prevent stopping of the machine."

    There is a note to the use of a CTE supply:

    "NOTE 1 On a centre-tapped earthed control circuit, the presence of one earth fault can leave 50 % voltage on a relay coil. In this condition, a relay can hold on, resulting in inability to stop a machine."

    Thus, there is the possibility of a CTE supply allowing an earth fault which can cause danger and thus be in contravention of 9.4.3.1.

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