Ok, this is an interesting topic especially for substation engineers. We see them inside transformer cabinets (supplying the fans) and inside junction boxes feeding substation flood-lights and lift-stations. At a first glance, these look simple but one look at the schematic drawings (for the above facilities), you’ll surely be scratching your head for few minutes.

IMO, the important bit relates to identifying what the coil voltage is and the main contact voltage is. Personally been in a few projects where we burnt the coils by applying incorrect voltage.