Coordination of standards

[stevenal]

Is there a reason why I'm hooking an ANSI C37.90 relay to a C37.06 breaker and to the same battery? The breaker will still open at 56% or 58% of nominal battery voltage, but the relay gave up long ago when it passed 80%? Is this just a case of the committees not bothering to speak to each other, or is there some real reason involved? They do agree on the maximums. What does IEC say about minimum voltage? C37.06 Table 8, note 2 would seem to apply the breaker voltages to protective relays as well. Yet I haven't found a relay that claims to meet this standard. What am I missing here?

 

[busbar]

That is a very good question. My guess is that the “0.58” value is mistakenly adopted from minimum AC-trip value for NEC-mandated minimums of low-voltage ground-fault breaker tripping, having nothing to do with DC-trip-voltage standards, and where there’s typically no DC trip sources for miles.

Although not referenced currently in NETA standards, the 0.577 value may have evolved minimum-AC trip-circuit voltage given AC-phase failure on control-power transfomers primaries—typically tapped immediately upstream of breaker AC-line terminals.

 

[dpc]

That's an interesting conflict. In the olden days, overcurrent relays got their power directly from the CT secondary current, so they would operate regardless.

In practice, I've found most breaker trip coils and lockout relays will operate well below 57% voltage.

I just looked at specs for a GE/Multilin SR-760 relay - its high range power supply will operate from 88 to 300 Vdc and the low range supply from 20 to 60 Vdc. So it would depend on your actual control power voltage. For a nominal 125 Vdc, the relay works down to about 70%. For a nominal 48 Vdc, the relay works down to 42%.