iigp2006:
I don't know if these are 'apart from the obvious' or not, but here are a few, anyway...
1. Interruption medium: Gas or Vacuum? What's the state of the art? What gives you the interrupting ratings that you need. Do all three on your list of suppliers offer both?
2. Insulation medium: gas or air?
3. The answers from (1) & (2) lead to maintenance and monitoring. Does the customer deserve the added grief of gas monitoring and topping off? Also, life expectancy and actual or simulated aging of the interrupters would most likely be the customer's No.1 question.
4. Space. How much room do you have? Have you drawn your oneline and added all necessary magnetics to achieve your protection and metering goals and asked the three manufacturers for their layout?. This is very important because brand 'A' for example is limited to say six CT's per breaker, wheras brand 'B, can install twelve. The same applies for PT's. The differences in size must be accounted for in your real estate.
5. Further to your oneline, you need to tell the manufacturers where are your cables entering and exiting the equipment top, bottom, side, and have them include that in their layout to see how large the equipment gets.
6. What auxiliary equipment is needed for installing the circuit breakers in place, what auxiliary equipment is required for moving the circuit breakers in the electrical romm? Do the breakers roll in and out on their own, or do you need some attachments? What does the customer prefer? Do you have space for storage of all that equipment?
7. Can the breakers be easily tested via switchgear built-in features, or do you need special equipment? That must be factored in the initial cost.
8. What maneuvering (sp?) is required to erect the equipment: forklifts, cranes, rollers, skates, other? Does your installation offer whatever the suppliers require?
9. How accessible are the instrument compartments for maintenance and troubleshooting? Do you need a six foot ladder to climb in and reach?
10. Are the bus joints in the main circuit and cable terminations easy to insulate? Are they applying removable boots, or do they require taping? Factor that in your installation cost.
11. Who provides the longest terminating distances for your MV cables? Does that match your terminators?
And the list goes on and on. But finally, please be advised that, worldwide, there are other manufacturers than the ones you've listed - some here in the USA.