It's a blast weapon, the (1) is the number of meters around the point of impact that is affected by the Haywire affect. So basically if you have 2 people in power armour standing shoulder to shoulder the 1 meter blast from that weapon has a good chance of disabling both of them.
The main thing to note, as a GM, when using this weapon is you need to decide where it hits and if someone is behind thin cover will will the EMP go through the that material to cause a Haywire affect without an actual hit. In my opinion, under most circumstances, this is a yes.
The haywire affect is on page 143 of the Death Watch core rule book.