FIGURE 6.3 Schematic diagram of properly wired variable autotransformers.

laboratory. Resistance devices used to heat oil baths should not contain bare wires.

Laboratory heating devices should be used with a variable autotransformer to control the input voltage by supplying some fraction of the total line voltage, typically 110 V, to the heating element of the device. If a variable autotransformer is not wired in this manner, the switch on it may or may not disconnect both wires of the output from the 110 V line when it is switched to the off position. Also, if this wiring scheme has not been followed, and especially if the grounded three-prong plug is not used, even when the potential difference between the two output lines is only 10 V, each output line may be at a relatively high voltage (e.g., 110 V and 100 V) with respect to an electrical ground. Because these potential hazards exist, whenever a worker uses a variable autotransformer whose wiring scheme is not known, it is prudent to assume that either of the output lines carries a potential of 110 V and is capable of delivering a lethal electric shock.

The external cases of all variable autotransformers have perforations for cooling by ventilation, and some sparking may occur whenever the voltage adjustment knob is turned. Therefore, these devices should be located where water and other chemicals cannot be spilled onto them and where their movable contacts will not be exposed to flammable liquids or vapors. Variable autotransformers should be mounted on walls or vertical panels and outside of hoods; they should not simply be placed on laboratory benchtops.

Because the electrical input lines, including lines from variable transformers, to almost all laboratory heating devices have a potential of 110 V with respect to any electrical ground, these lines should always be viewed both as potential shock hazards and as potential spark hazards. Thus, any connection from these lines to a heating device should be both mechanically and electrically secure and completely covered with insulating material. Alligator clips should not be used to connect a line cord from a variable autotransformer to a heating device, especially to an oil bath or an air bath, because such connections pose a shock hazard. They also may slip off, creating an electrical spark and, perhaps, contacting other metal parts to create an additional hazard. All connections should be made by using, preferably, a plug and receptacle combination, or wires with insulated terminals firmly secured to insulated binding posts.

Whenever an electrical heating device is used, it is essential to use either a temperature controller or a temperature-sensing device that will turn off the electric power if the temperature of the heating device exceeds some preset limit. Similar control devices are available that will turn off the electric power if the flow of cooling water through a condenser is stopped owing to the loss of water pressure or loosening of the water supply hose to a condenser. Fail-safe devices, which can be either purchased or fabricated, can prevent the more serious problems of fires or explosions that may arise if the temperature of a reaction increases significantly because of a change in line voltage, the accidental loss of reaction solvent, or loss of cooling. Fail-safe devices should be used for stills employed to purify reaction solvents, because such stills are often left unattended for significant periods of time.

(See section 6.C.1 for additional information.)

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