GB1129359A - An electrical measuring instrument - Google Patents

Abstract

1,129,359. Electric meters. LEYBOLD HOLDING A.G. Sept. 9, 1966[Sept, 17, 1965], No.40480/66. Heading G1Q [Also in Division H1] A multiple range electric meter incorporates a safety switch which must be operated before the range can be changed. The safety switch when operated either isolates the meter from the circuit under test by braking a series connection, or protects the meter by placing a short circuit across it, or both. The need for such a safety switch is stated to arise particularly when it is proposed to switch from a sensitive voltage range to a sensitive current range for use of the latter as an ultra-sensitive volt meter. In such circumstances one may switch through the less sensitive current ranges and so overload either the meter or the circuit under test. Similar safety provision is also stated to be desirable in meters with power measuring ranges. Figs. 2 and 3 show a push-button safety switch whose operating plunger 5 normally engages one of the cam surfaces 511 on a cam plate 51 carried by the shaft of the rotary range selector switch 3, and so locks this switch. When the safety switch plunger is depressed, a groove 53 in it moves into register with the cam plate and so releases the lock on the selector switch. Alternatively both switches may be rotary and each carry a radially slotted cam disc, the cams rotating in perpendicular planes but each requiring that a slot in the other first be moved into the plane of its own rotation. It is also envisaged that operation of the safety switch may release a magnetically or electrically actuated lock on the range selector switch. Fig. 1 (not shown) depicts the front panel of an instrument fitted with such a pair of switches. There are two pairs of input sockets (4a, 4b) one for current and one for voltage measurement. When the safety switch is operated its contacts 6, 7 respectively short circuit the current input pair and isolate the voltage input.