Trigger Voltage
As you might have known, the flash tube is filled with gas. While the flash is being charged, the charge current is limited and the gas becomes deionized. At the charging voltage, the internal resistance of the gas in the flash tube is high enough to prevent any flow of current from the main capacitor. However, by applying a trigger voltage, the gas will be ionized and becomes conducting, allowing the main capacitor to discharge rapidly through the gas. As a result, a brief flash of light is generated. This trigger voltage is usually a short pulse of 5-15kV.
A trigger voltage is frequently generated by a spark coil mechanism which is usually triggered by the shutter contacts in the camera. This voltage is positioned on the low-voltage side of the spark coil so that it only appears on the flash contact. In this way, the user will not be injured accidentally, and the camera will not be damaged. This is especially important because digital cameras have circuits very sensitive to high voltage and strong current.
Some older flashes are known to have very high trigger voltage that may damage the circuits of a digital camera. Fortunately, most modern dedicated flashes have low to very low trigger voltages that can be safely used on digital cameras. Click here for a rather complete list of the trigger voltage of many flashes, studio strobes included. If you suspect your flash may have a very high trigger voltage, you may consider to use a simple device called safe sync. Click here for the details. Keep in mind that, with a safe sync, the TTL capability of your flash becomes not available because safe sync only supports ISO hotshoe and PC cord/terminal. Fortunately, virtually all new Nikon TTL-compatible flashes have low trigger voltage and should be safe.