A component of a good CT's characteristic performance is a low admittance value. This admittance value can be measured while in-circuit using the 505B (pictured). The 505B is placed in series with an in-circuit CT and it superimposes a small 1.6kHz signal onto the live 50 or 60Hz secondary metering circuit. A filter on board the 505B extracts the 1.6kHz admittance reading and from this a quick determination is made on the CT's general health. Admittance testing only adds an extra 10 minutes to a polyphase meter test and is quickly becoming part of a routine test requirement for some utilities.