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Normally, a current transformer drives a current into an ammeter, and it can be used in this way. If you want to use a voltage-sensitive device (which the ADC input is), you need to use a burden resistor to develop a voltage.

The 30 A c.t does indeed have a built-in burden. In fact I believe, but only on circumstantial evidence, that the 30 A and 100 A devices are one and the same, the only difference being the 30 A version has a burden resistor connected inside the casing. The burden is the right value to give you the stated voltage at the stated current. (I put it that way because people who have taken them apart report different values, so I imagine different batches have slightly different number of turns and this is to compensate).

The 100 A version has two zener diodes instead to protect against high voltage appearing on the plug - not necessary with a permanently connected burden.

If you have unexplained problems with this 30 A version, some users have found the burden resistor is either damaged and faulty or badly soldered, so be aware of this.

Thanks a lot Robert for shedding some light on to the matter.

One last thing. The burden is the right value to give you the stated voltage at the stated current?

The datasheet sets the output as voltage/1V. What exactly does this mean? Does it mean that it gives you a per unit value? And that max current gives an output of 1 per unit?

The "glossy" (it's not really a data sheet) was written and translated into some semblance of English. They actually mean you get 1 V rms output for a primary current of 30 A rms - subject to manufacturing tolerances of course.

(My notes say the internal burden is 72 Ohms, which makes the secondary turns 2160).

Thanks a lot Robert, you really saved me a lot of time and trouble explaining everything so in depth.