Voltages
Internal camera circuit shorts center contact and shoe mount, thus triggering the flash. The trigger voltage (for a flash) between the center contact and the shoe have varied over the years, between manufacturers, and even in the same manufacturer. When the contacts with a shutter were mechanical contacts, the actual voltage did not matter too much as long as it did not cause arcing, but now with electronic triggering, it can cause problems.
The ISO 10330 specification allows for a trigger voltage of 24 volts, some manufacturers, particularly Canon, ask for no more than 6 volts. Some older flashes may have a high voltage, sometimes in the hundreds of volts.
It is possible to connect a high voltage triggering flash with a camera which can only tolerate 6 volts, through the use of an adaptor which isolates the two units. Also many radio triggers, e.g. PocketWizard, while giving a low voltage to the camera, can handle 200V from the flash port thus isolating the camera from the flash's high voltage trigger.
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