6.1.3.6 Minimum contact requirementsThe minimum requirement for the busbar replica is the record of the disconnector positionby using just one auxiliary contact, either NO or NC type. However recording a pair ofauxiliary contacts, representing the OPEN and CLOSE position, offer additional featureswhich can improve the reliability of the bus replica including supervision possibilities.6.1.3.7 Auxiliary contact evaluation logicTwo logic schemes can be found.Scheme1_RADSS "If not OPEN then CLOSED"As the name of the scheme suggests, only when the auxiliary contacts signal clean openposition ("normally open auxiliary (NO) contact input" = inactive and "normally closedauxiliary (NC) contact input" = active), the disconnector is taken to be open. For all othersignal combinations the disconnector is considered to be closed. This scheme does notpose any special requirements to the auxiliary contact timing. Only the disconnector NCcontact must open before the disconnector main contact is within arcing distance. Thetime during which the OPEN and CLOSED signal inputs disagree (that is, both binaryinputs are active or both are inactive) is monitored by the isolator supervision function.The maximum time allowed before an alarm is given can be set according to thedisconnector timing.Scheme2_INX "Closed or open if clear indication available otherwise lastposition saved"As the name of the scheme suggests, only when the auxiliary contacts signal clean OPENor clean CLOSED position disconnector is considered to be open respectively closed.However this poses the stringent requirements on the auxiliary contacts that the CLOSEDsignal must become active a certain time (>150 ms) before current starts flowing forexample, through arcing. Otherwise this current will not be taken into account in thebusbar protection and this can result in a maloperation. Therefore, good timing of twoauxiliary contacts is definitely required.The time during which the OPEN and CLOSED signals disagree (that is, both binaryinputs are active or both are inactive) is monitored by the isolator supervision function forboth of the above two schemes. The maximum time allowed before an alarm is given canbe set according to the disconnector timing.Table 12 and the following two figures summarize the properties of these two schemes.Section 6 1MRK 505 337-UUS -Differential protection108Application manual
