An electrical combination , comprising: a power tool including: a motor ; a power tool terminal connected to the motor; and a power tool detection terminal connected to the power tool terminal; and a battery pack including: a battery pack terminal for connecting with the power tool terminal; characterized in that the battery pack further includes a battery pack detection terminal for connecting with the power tool detection terminal; a battery which is electrically connected with a first potential point and a second potential point which has different potential from the first potential point with the battery pack detection terminal being connected to the first potential point; a first detection circuit for connecting the battery pack terminal to the second potential point; and a second detection circuit for connecting the battery pack detection terminal to the second potential point; and a controller operable to detect a first voltage detection signal from the first detection circuit, detect a second voltage detection signal from the second detection circuit, and output a control signal to cut off the connection between the battery and battery pack terminal when the difference between the first voltage detection signal and the second voltage detection signal is greater than a predetermined value. a power tool including: a motor ; a power tool terminal connected to the motor; and a power tool detection terminal connected to the power tool terminal; and a motor ; a power tool terminal connected to the motor; and a power tool detection terminal connected to the power tool terminal; and a battery pack including: a battery pack terminal for connecting with the power tool terminal; characterized in that the battery pack further includes a battery pack detection terminal for connecting with the power tool detection terminal; a battery which is electrically connected with a first potential point and a second potential point which has different potential from the first potential point with the battery pack detection terminal being connected to the first potential point; a first detection circuit for connecting the battery pack terminal to the second potential point; and a second detection circuit for connecting the battery pack detection terminal to the second potential point; and a battery pack terminal for connecting with the power tool terminal; characterized in that the battery pack further includes a battery pack detection terminal for connecting with the power tool detection terminal; a battery which is electrically connected with a first potential point and a second potential point which has different potential from the first potential point with the battery pack detection terminal being connected to the first potential point; a first detection circuit for connecting the battery pack terminal to the second potential point; and a second detection circuit for connecting the battery pack detection terminal to the second potential point; and a controller operable to detect a first voltage detection signal from the first detection circuit, detect a second voltage detection signal from the second detection circuit, and output a control signal to cut off the connection between the battery and battery pack terminal when the difference between the first voltage detection signal and the second voltage detection signal is greater than a predetermined value. <EOS>