Before entering into a consideration of the individual devices that concern us, it would be as well to explain some of the associated technology/ terminology.
The switch is possibly the most elementary device in the field of electronics. A switch controls the flow of current in a circuit in a manner such that either the current flows at a value determined by the other components in series with it, or does not flow at all, as the case may be. However this ideal behavior is actually never exactly what is seen in real life. A switch has it's own parameters that determine how fast it can switch from open to closed, or how rapidly it can interrupt the flow of current once it is has been opened. Also of course there are more elementary considerations such as the current handling capacity of the switch and the peak voltage it can cope with before damage or other unwanted effects occur.
Mechanical switches such as are common in the home are in actuality far from ideal in their behavior. The time taken to switch from off to on ( the commutation time) is typically in the millisecond range. Also spurious effects such as bouncing may occur as the switch fluctuates rapidly from open to closed in the process of being physically manipulated by the operator.
Electromagnetic relays and reed switches experience similar problems to those seen in the humble light switch. Long commutation and switch bounce are standard features of virtually all mechanical switching devices.
With the advent of transistors and similar devices such as thyristors one would have thought that these slow switching problems would be things of the past. This is in fact largely true. But semiconductors are limited in other ways, it is very hard to find semiconductors capable of switching many kiloamperes especially at potentials in the kilovolt region, and those devices that can manage high currents such as the larger thyristors are troubled by overly high commutation times. Whilst there are now semiconductors coming onto the market capable of performing at these extremes of current and voltage there are some requirements which put even these devices to shame. If you want to switch 50 kilo Amperes with a sub 20 nanosecond commutation time at 20kV you are going to be in trouble if you are relying on semiconductor technology. However there is an alternative class of devices that have been around long before the humble transistor came on the scene. You might think that vacuum tubes and similar are a thing of the past. But for problems of this magnitude they are the only things on the market that will do the job.
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