IEEE C37.2-1991 pdf free.IEEE Standard Electrical Power System evice Function Numbers.
Auxiliary,Position,and Limit SwitchContacts.The letters a and b shall be used forall auxiliary,position，and limit switch con-tacts for such devices and equipment as circuitbreakers,contactors，valves and rheostats,and contacts of relays:
Contact that is open when the main de-vice is in the standard reference position,commonly referred to asthenonoperated or de-energized position,and that closes when the device assumesthe opposite position.
Contact that is closed when the main de-vice is in the standard reference position,commonly referred to as thenonoperated or de-energized position,and that opens when the device assumesthe opposite position.
The simple designation a or b is used in allcases where there is no need to adjust the con-
tacts to change position at any particular pointin the travel of the main device or where thepart of the travel where the contacts change po-sition is of no significance in the control or op-erating scheme.Hence the a and bdesignations usually are sufficient for circuitbreaker auxiliary switches.
Standard reference positions of some typicaldevices are given in Table 1.
Auxiliary Switches With Defined Op-erating Position. When it is desired to havethe auxiliary, position,or limit switch desig-nation indicate at what point of travel the con-tacts change position，as is sometimesnecessary in the case of valves and for othermain devices,then an additional letter (or apercentage figure,if required) is added (as asuffix to the a or b designation) for thispurpose.
For a valve, the method of designating suchposition switches is shown in the diagram andlegend in Fig 1.There are thus two points toconsider in visualizing or describing the op-eration of these position switches.The first iswhether the contact is a or b as indicated by thefirst letter. The second is where the contactchanges position, either at or near:
(1)The closed position of the valve c,
(2) The open position of the valve o, or
(3) specified percentage such as 25% of the full open position, for example, a26.IEEE C37.2 pdf download.