Lockout Relays

Low impedance relays are generally current-operated. For lockout relay applications, especially breaker tripping for transformer protection, high-speed operation is vital, and this can be more consistently and accurately achieved with a voltage-operated (high impedance) relay. Read the descriptive bulletin you referenced - it explains the function pretty clearly.

But high speed operation can be achieved through high current, i.e. low impedance relays.

because the design engineer golfs once a week with the high impedance relay sales guy

The lockout relays are short time rated voltage operated relay, i.e if auxiliary voltage is 110 volts, the continious relay rated voltage will typically be 50%. This is to acheive high speed of operation. For the same reason, the voltage to the coil will be cutoff through its own contact once the relay operates to prevent burnout of the coil.

The relays must operate on the DC battery banks (usually 125VDC or 250VDC) as other relays do for protection of circuits. Sometimes the distance from the relay may be quite a long distance within a large utility or plant. This is the way it WAS done. It worked so why change it? They are designed so that the various transients within the system do not cause false trips or no trips at all. I remember when some battery company tried to sell NiCad batteries for these systems. The voltage was there, but the natural transient absorbing action of the old lead acid batteries was not. Lots of false trips. The only choice was to remove the new batteries and put the old type back. A protective circuit must be reliable, fast, and not error prone. It is possible for changes to be made but the research for those changes must be flawless and extensive. I have no idea whether low impeadance relays would answer all criteria, but I question the motive for using them. What is the advantage? Change for change sake is not an advantage unless there is an improvement that is gained. We are talking multiple millions of dollars in most electrical system with lives at stake if errors are made. Not to be taken lightly without proper investigation.