Electrical safety switch for electrical appliances

An electrical safety switch for protecting an electrical appliance requiring a ground, an on/off switch disposed in series with a resistor and primary coil of a transformer between the ground line and the B+ line of the appliance, and a triac connected in series with the load of the appliance between the B+ line and the neutral line with the gate triggering the triac connected to the transformer to be energized thereby upon a current being present through the on/off switch.

DESCRIPTION 
Technical Field 
This invention relates to an improved circuit for electrical appliances, 
and in particular, to an improved electrical safety switch for appliances 
which will protect the operator in the event there is not a proper ground 
provided in the electrical outlet to which the appliance is connected. 
Background Art 
The grounded cord or the three-wire cord on electrical appliances was 
established for the protection of the operators against electrical shock 
while operating the appliance or handling the appliance. The need for the 
three-wire cord and the properly grounded electrical outlet which was 
established in the electrical code was meant to assure that the electrical 
outlet was properly wired and that the neutral receptacle wire of the cord 
is connected to the neutral wire at the outlet. Due to the fact however 
that a great many electrical outlets are improperly wired or that a fully 
ground line is provided which does not establish a proper ground line for 
the appliance it is necessary that the on-off switch to operate the 
appliance be provided with a safety circuit such that the appliance will 
not operate unless a ground is established to the appliance and unless the 
outlet is properly wired as to establish the proper earth ground, the 
proper position of the B+ line and the proper positioning of the neutral 
line at the electrical outlet. 
One such protective switch is disclosed and illustrated in U.S. Pat. No. 
4,028,594, issued June 7, 1977, to one of the inventors of the subject 
application. This earlier protective switch provided the assurance that 
the electrical outlet was properly wired and that a ground was provided to 
the electrical appliance before it could be energized. The device was 
designed to pass current through the ground line sufficient to energize a 
switching device or relay. Since ground fault protective devices were 
placed on the market for installation with the electrical outlet to 
protect operators of electrical appliances from hazardous electrical 
shocks it became necessary to not merely provide a solid state triggering 
device or switch but to reduce the amount of current applied to the ground 
line to a level considerably below that protected by a ground fault 
device. These ground fault protective devices were not triggered unless 
there was a minimum of five milliamps of current at 115 volts passing 
through the ground fault protective device causing the circuit to the 
electrical outlet to be opened and protect the operator. These systems are 
an aid to protection of the operators, but nonetheless, they subject the 
operators from time to time to needless electrical shocks which are 
uncomfortable and distracting to normal working procedures. They do not 
protect the operator from an improperly wired appliance. 
The safety switch of the present invention insures the appliance and the 
outlet are wired properly, including earth ground, before the appliance 
will operate. 
Due to the fact that the ground fault protective device would trip with the 
amount of current utilized to energize the device described in prior U.S. 
Pat. No. 4,028,594, the two devices could not function when utilized 
simultaneously in the same circuit. Both devices make valuable 
contributions to electrical safety and it would be advantageous to permit 
them to function in combination with each other. The present invention 
provides an improved safety switch which accomplishes this purpose and 
affords a second continuous monitoring of earth ground to the appliance. 
This improved device functions well with one-fifth (or less) of the 
allowable current approved and deemed safe for the ground fault safety 
devices now on the market. Additionally, this improved device cannot be 
negated or by-passed for convenience by utilizing 2 wire adaptor plugs on 
3 wire grounded appliance cords. To circumvent the safety of the improved 
device would necessitate disassembly of the safety protected appliance and 
revamping of the circuit. The ground fault interrupter turns off current 
to an electrical appliance when there is any current leaks, whereas the 
improved device does not allow the electrical appliance to turn on if 
there is not a proper earth ground circuit provided. The appliance will 
not operate if ground is not present and will cease to operate if ground 
is lost during operation. 
The electrical protective switch of the present invention combines a 
transformer between the B+ line and the ground line such that if the 
operating switch is closed a very small monitoring current is applied to 
the ground line through one winding of a transformer and a voltage is 
applied to a triac such that a circuit is closed through the load of the 
appliance to energize the same and to carry the current load until the 
operating switch has been opened. A lamp on the appliance in circuit with 
the safety circuit will tell an operator, after the plug is placed in the 
outlet, that there is current in the outlet. If after the on/off switch is 
moved to the on position and the light stays on, the light shows there is 
not a proper ground to the appliance or the outlet is improperly wired, 
i.e., neutral and B+ is reversed in the outlet. The safety switch of the 
present invention does not place more than one milliamperes of current 
into the ground line such that it will not trip a ground fault system 
utilizing the five milliamperes of current. 
DISCLOSURE OF THE INVENTION 
An electric safety switch for use with appliances to assure that the 
appliance is properly connected to earth ground, the B+ line and the 
neutral line of an electrical outlet before it will operate. The 
electrical safety switch of the present invention comprises a primary 
transformer coil connected in circuit between the B+ line and the 
activating switch for the appliance and the ground line for the electrical 
circuit. A second coil for the transformer is connected between the 
neutral line and the gate of a triac to place the triac in conduction. The 
input and output gates of the triac are positioned between the neutral 
line and the load of the appliance which is also directly connected to the 
B+ line. Suitable resistors are established in the circuits of the primary 
and secondary windings of the transformer to reduce the current through 
the primary winding of the transformer to an amount not exceeding one 
milliampere.

BEST MODE FOR CARRYING OUT THE INVENTION 
The electric safety switch for installation in appliances according to the 
present invention is disposed in a three-wire cord 6, which cord, between 
the plug 7 and the load 8 of the appliance, has a ground line 9, the 
neutral line 10, and the B+ line 11. 
The safety circuit for energizing the load 8 when the plug 7 is inserted in 
an electrical outlet which is properly wired allows current from the B+ 
line 11 to flow through the load 8 and back though the neutral line 10, 
maintaining the load energized until the subsequent opening of the on/off 
switch 12 connected to ground line 9. The electrical safety circuit of the 
present invention comprises a transformer generally designated 14 disposed 
with the primary winding 15 connected via lead 16 at one side to the B+ 
line 11 and connected via lead 17 through a resistor 18 to one side of the 
on/off switch 12 via lead 19 which is also connected to the ground line 9 
via lead 20. The transformer 14 is preferably a Triad F-94X. The 
transformer 14 has an output coil 21 connected between the neutral line 10 
via lead 22 to a resistor 24 via lead 25. The resistor 24 exceeds 1000 
ohms and imposes a small voltage via lead 26 onto the gate 27 of a triac 
28. The small voltage which has a maximum of about 2.5 volts of 3.0 
milliamperes will trigger the triac such that the triac closes a circuit 
from B+ line 11 through the load 8 and conductor 32 and 33 to the primary 
of the triac and through the triac to the neutral line 10 via lines 31 and 
30 to energize the load. The circuit is normally open due to the inherent 
characteristics of the triac. The parallel connection with the triac of a 
resistor 35 and a condensor 36 connected via leads 37 and 38 to the leads 
32 and 30 reduces current surges when the triac turns on and insures 
correct operation of the triac. 
A lamp 40 is also disposed in parallel with the triac such that the lamp 40 
is on when the plug is in an outlet where power is available to the 
appliance and the triac is open indicating the switch is off or the proper 
ground is missing. If proper ground is available and the outlet is wired 
properly, closing switch 12 will cause the triac 28 to close and 
extinguish the lamp 40. The lamp is connected to leads 30 and 33. 
The transformer circuit provides a very small monitoring current not 
exceeding 1 milliampere continuously through the ground line. This 
condition is afforded with the plug 7 establishing electrical contact in 
an outlet provided with 115 volts. If the earth ground is subsequently 
lost the appliance will shut off as the triac will open, interrupting the 
circuit to the load. 
The representative values of the various components of the electrical 
safety switch as illustrated to afford less than 1 milliampere, e.g. 0.985 
milliampere, in the monitoring circuit through the on/off switch is as 
follows: 
______________________________________ 
Switch 12 Single pole, single throw 
Transformer 14 Triad F-94X 
Resistor 18 116.8K ohms 
Resistor 24 1600 ohms 
Triac 28 Motorola 2N6072B 
Resistor 35 100 ohms 
Condenser 36 0.1 Microfarads 
______________________________________ 
A circuit constructed as disclosed in the drawing with the components 
described can carry a maximum load of four amperes to the load 8 of the 
appliance. The load 8 will be the power plant for an appropriate appliance 
or the load 8 may be a secondary switching device when the appliance 
requires more than four amperes. 
Having thus disclosed the invention with reference to a preferred 
embodiment it is to be understood that changes may be made to scale the 
same up or down in accordance with good engineering practice and will not 
detract from the scope of the invention as described in the appended 
claims.