Radiography position of the exposure switch should be as follows:

Especially for fixed equipment, at the jurisdiction panel or at the locale deliberate to be occupied by the operator; and for mobile and portable equipment, such as to enable the operator to be exterior the radiation beam and to be not less than 2m from the tube housing and from the patient.

X-ray units that you are referring to are intraoral units, which are sketch to take x rays of a few teeth at a time. They are mounted on the wall, and the arm extends over to the patient for the x ray. It is very common for these units to have exposure switches mounted outside of the room, usually in a hallway, with a push-button switch as you mentioned. The exposure switch is mounted outside the room so that the operator is in a shielded area and not in the room with the x-ray unit for all exposures.

Whenever radiation can be generated without connecting the paraphernalia to the electrical mains supply; or when connected to the electrical mains supply if the equipment requires such a connection to be made for battery charging Control of Exposure Duration.

For radiography, there should be a means for terminating the exposure automatically after a pre-set time, electrical charge (mAs) or quantity of radiation or after a tomographic scan has been completed. To guard against failure, an additional means of termination should be provided which is independent of the normal means. The release of an exposure switch may be regarded as the additional means

For fluoroscopy, the release of an exposure switch should be regarded as the normal means of termination. An additional means of termination should be provided which operates automatically when a predetermined integrated fluoroscopy time not exceeding 10 minutes has elapsed. This device should give an audible warning at least 30 seconds before termination to enable the operator to reset the device if the exposure needs to be prolonged.

Equipment for computed tomography should have a device which terminates the exposure automatically as soon as the selected scan has been completed or scanning stops before completion. Initiation of an exposure should not be possible if the scanning motor fails to start unless "warm up" conditions or a localization exposure have been selected.

If a "warm up" facility is provided on computed tomography equipment there should be a clear indication on the control panel when the "warm up" mode has been selected and there should be a device which de-energies the X-ray tube on completion of the desired "warm up" phase.