Patent GB-1480535-A (GB)
Title: Power line fault indicating system
IPC: G01R31/58
Filed: 1975-06-11
Abstract: 1480535 Monitoring electrical apparatus BAXTER TRAVENOL LABORATORIES Inc 11 June 1975 [29 July 1974] 25124/75 Heading G1U An A.C. power line fault indicator for connection to a three-wire supply includes an oscillator comprising a neon lamp connected across a capacitor which is itself in series with a resistor, the series connection being supplied via a rectifier from the live and neutral input conductors of the indicator, and also includes a switch across the  resistor, which switch is controlled by means detecting any faults in the supply, the oscillator thereby changing its operation depending on the state of the switch to provide steady illumination of the neon lamp. As shown in Fig. 1, the power supply to an artificial kidney machine 12 is monitored. Conductors 19, 20, 21 are connected e.g. by a plug and socket to the live, neutral, earth conductors of the power supply. Under normal, fault-free conditions, 240 volts D.C. is supplied by the power supply Rl, R2, CR1, C1, R3 to energize the oscillator. A second supply R4, Rs, CR2, C2 supplies 12 volts D.C. to the sensing logic and drives circuitry on line 24. Since, normally, the neutral line will be substantially earth, Q1 is turned on, reducing its collector voltage to almost earth, thereby inhibiting conduction through Q2 which acts as a Zener diode if reverse biased by around 7À0 volts. Thus Q3 is also off, capacitor C3 charges up to 12 volts, Q 4 , Q 5  are on and resistor R 15  is by-passed. The period of the relaxation oscillator is thus determined by R 6 , R 14  and C 4  giving a frequency around 20 Hz which appears as a continuous glow. If the machine 12 is inter-connected to the supply incorrectly, i.e. live 14 connected to machine earth 17 or neutral 15 with 13 disconnected, or live not connected, or neutral unconnected, no D.C. supplies are produced and the lamp 22 remains dark. With faults in the power supply itself, the lamp 22 is arranged to flash as follows:- Open circuit or high impedance earth 18 Q 1  clearly remains off and current therefore flows through the base collector junction of Q 1  and through R 8  to Zener transistor Q2, thus turning on transistor Q3. This discharges the capacitor C 3  turning off Q 4 , Q 5  and inserting R 15  in the oscillator circuit. The oscillator frequency now changes to around 1À5 Hz, causing the lamp 22 to flash. If the earth lead 18, 21 is a high impedance above a predetermined level, the leakage current will cause the collector voltage of Q I to be high enough to Zener the transistor Q2, thereby indicating a fault as before. Live and neutral interchanged or excessive neutral voltage In this case, the D.C. supplies are still provided and Q1 is therefore on. When the neutral wire 20 undergoes a positive excursion Q3 is off. When the neutral is negative, current flows from ground at the collector of Q1 through Zener Q2, to turn on Q3. The on/off period of Q3 is insufficient to charge C3 to the triggering point of the drivers Q4, Q 5  and a fault is signalled. In certain circumstances, a small A.C. can exist on the neutral wire 18, e.g. due to unbalanced loads or earth loops. If the amplitude thereof is high enough, or the negative half-cycle a similar situation to above occurs and Q3 comes on, lowering the average level of voltage or C3 and signalling a fault. Modified embodiments are illustrated in Figs. 2 and 3 (not shown) where the logic sensing circuitry is arranged to allow setting of the acceptable levels of leakage, earth impedance etc. The logic signal indicating a fault, across C 3 , may be utilized to shut-down the monitored machine.