class 2 type transformers

[pilot]

hi everyone
I have had a question asked ,exactlly what is a class 2 transformer? and what must it be fused at (primary or secondary)?

thanks a lot

[bigrockk]

A few excerpts from the 1998 code book:


16-006 Class 2 Low-Energy Power Circuits. Circuits which are neither remote-control circuits nor signal circuits but in which the current is limited in accordance with Rule 16-200 shall be classed as low-energy power circuits and shall be considered to be Class 2 circuits.

16-200 Limitations of Class 2 Circuits (see Appendix B)
(1) Class 2 circuits, depending upon the voltage, shall have the current limited as follows:
(a) 0 to 20 V. Circuits in which the open-circuit voltage does not exceed 20 V shall have overcurrent protection rated at not more than 5 A, except that overcurrent protection shall not be required where the current is supplied from:
(i) Primary batteries which under short circuit will not supply a current exceeding 7.5 A after 1 min; or
(ii) A Class 2 circuit transformer; or
(iii) A device having characteristics which will limit the current under normal operating conditions or under fault conditions to a value not exceeding 5 A; or
(iv) A device having a Class 2 output;
(b) Over 20 V but not exceeding 30 V. Circuits in which the open-circuit voltage exceeds 20 V but does not exceed 30 V shall have an overcurrent protection rating not exceeding 100/V amperes, where V is the open-circuit voltage, except that the overcurrent protection shall not be required where the current is supplied from:
(i) Primary batteries which under short circuit will not supply a current exceeding 5 A after 1 min; or
(ii) A Class 2 circuit transformer; or
(iii) A device having characteristics which will limit the current under normal operating conditions or under fault conditions to a value not exceeding 100/V amperes, where V is the open-circuit voltage; or
(iv) A device having a Class 2 output;
(c) Over 30 V but not exceeding 60 V. Circuits in which the open-circuit voltage exceeds 30 V but does not exceed 60 V shall have an overcurrent protection rating not exceeding 100/V amperes, where V is the open-circuit voltage, except that the overcurrent protection shall not be required where the current is supplied from:
(i) A Class 2 circuit transformer; or
(ii) A device having characteristics which will limit the current under normal operating conditions or under fault conditions to a value not exceeding 100/V amperes, where V is the open-circuit voltage;
(d) Over 60 V but not exceeding 150 V. Circuits in which the open-circuit voltage exceeds 60 V but does not exceed 150 V shall have an overcurrent protection rating not exceeding 100/V amperes, where V is the open-circuit voltage, and in addition shall be equipped with current-limiting means other than overcurrent protection, which will limit the current, either under normal operating conditions or under fault conditions, to a value not exceeding 100/V amperes, where V is the open-circuit voltage.
D (2) A device having energy-limiting characteristics may consist of a series resistor of suitable rating or other similar device.
D (3) A Class 2 power supply shall not be connected in series or parallel with another Class 2 power source.
16-202 Methods of Installation on Supply Side of Overcurrent Protection or Transformers or Other Devices for Class 2 Circuits. In Class 2 circuits, the conductors and equipment on the supply side of overcurrent protection, transformers, or current-limiting devices shall be installed in accordance with the requirements of other appropriate Sections of this Code.