How many 85 relays (pilot wire) are needed if load is fed with 4 cables? 2

[Dvhez]

I'm feeding a Switchboard from a Switchgear through 4x(1x3c 500 MCM) and each cable will carry 160 A to feed this switchboard (542 kVA - 600 V). These are mining cables with pilot wires so I assume the Switchgear needs to consider a Pilot Wire Relay (function 85)... but that's for only 1 conductor right? So I'll need to consider 4 different relays at the switchgear?

Thanks!

 

[LionelHutz]

Part of the function of the pilot circuit is to ensure the cable isn't cut or disconnected leaving an exposed live end. So, one pilot relay per cable.

 

[oldfieldguy]

Thinking back to GE pilot wire relays I worked with in the past, you need ONE pair of relays.

I worked on systems for in-plant industrial distribution at the 15 kV level where multiple conductors per phase were the norm. I also worked with them in an transmission-distribution system where we protected several short-length 69 kV transmission lines between close-together substation.

A CT for each phase at each end feeds into its corresponding relay. There, conversion takes place to the actual signal-level current that passes over the pilot wire for comparison from end to end, basically a long-reach differential scheme. The comparison current is an AC waveform. On the old GE scheme, a DC signal was piggy-backed onto the pilot wire and provided integrity check functions. If the pilot wire failed, the relays then reverted to a gross-order overcurrent function.

They're slowly (like much in the utility biz) being phases out in favor of smart relays which can replicate that function and much more with modern communications.



old field guy

 

[LionelHutz]

I read mining cables and concluded that meant the cables were portable trailing cables. If so, then a relay like a Startco SE-105 would be used to do the ground fault and ground check functions normally required with mine trailing cables. Otherwise, something completely different or possibly even nothing at all is needed.