Blocking Diode Bypassing 86 Lockout Relay

[enterprisenx]

Hello,

It was recently discovered that some years ago, a modification was made to several switchgear that involved the installation of a diode in a tripping circuit to essentially bypass the 86 relay to trip the circuit breakers. However, the 86 relay was still needed to be reset to re-energize the system. I can't figure out why this was done other than assuming the engineer at the time had issues with the mechanical 86 relay that was being used and decided to ensure both feeder breakers tripped by adding this additional wiring. All relays in the switchgear are electro-mechanical.

Has anyone seen this done before? Does it make sense to leave it in or does it create problems?

Please see attached. We have two 12kV feeder breakers (separate feeders) that supply a 12kV/480V transformer & 480V switchgear. There is only 1 - 86 LO relay. Perhaps there should have been 2 LO Relays. Anyway, just wanted to get peoples' perspectives on this installation as I have not seen diodes used to bypass or otherwise circumvent control functions.

Thanks,

Brian

 

[gar]

190725-0834 EDT

enterprisenx:

I think "blocking diode" is the wrong term to use. Rather my guess is that it is being used as a logic function, and in this case means we need to look at when it conducts.

I am not in your business and thus the circuit does not make sense to me.

It is obvious that the circuits are 48 VDC with + being at the top, and its common (-) is at the bottom.

Are all the three 48 V common buses connected together? If not, then I have a problem understanding any logical action of the diodes. I don't need the +48s to be connected together. But if they are not, then there can be problems with different voltages.

Assume commons are connected together.

If the logic in the center section energizes relay coil 86, then if that coil voltage is large enough, then the left diode will conduct thru contacts 2C, 4C, and coil 52 to energize coil 52 independent of any condition of the basic logic on the left above the point where the diode connects.

This might shorten the time to trip 52, and/or make sure 52 trips if coil 86 is energized. But obviously contacts 2C and 4C must be closed.

.

 

[gar]

190725-1005 EDT

enterprisenx:

Looking further at your circuit I see that both the upper logic areas have an 86 contact in their circuit.

Thus, I believe the purpose of the diodes was to shorted the time to trip of the coils 52. Coils 52 are energized at the same time as 86, and are not delayed by the time for 86 to close.

But note that if a coil 52 was energized by something other than 86 that would not cause the other coil 52 to be energized. In this sense the diodes are blocking. But I don't view that as a reason to call the diodes blocking.

Fundamentally the diodes are being used as an "OR" logical function.

.

 

[SG-1]

I see the tripping circuit of an automatic transfer system on the left & right hand sides. (see the under voltage relay contacts - 27) Another clue is the 43A contact, (Selector switch to place the system in Automatic Mode.)

The over current protection (51/51N) is required to trip the lockout relay (86) & block any farther operation of the automatic transfer.

The under voltage relays (27) must not trip the lockout relay (86) only the medium voltage breaker (52) You can see the 52TC (trip coil) on each side. The "blocking diodes" stop the under voltage trip signal from the 27s from reaching the lockout (86) coil" & disabling the system.

Gar you are correct that the diodes speed up the tripping. A modern lockout adds about 3 cycles to the clearing time. More important is that the diodes stop the 27 from operating the lockout.

 

[petersonra]

looks to me like they want the 52 trip to work just for that unit while the logic in the center of the drawing trips both. The blocking diodes allow voltage to flow from the center section logic but not from the right hand 52 to the left hand 52.

 

[enterprisenx]

I have included another photo showing the single-line for the income feeders to the LV transformer. Downstream of that is just the 480V switchgear.

This is a protection control circuit for switchgear. I am aligned that it looks like it shortens the the time to coordinate a trip between both feeder circuit breakers. With just the 86LR, you need that to close it's contact to allow current flow to trip the coil for the other circuit breaker.

Is it uncommon to rely on a single 86LO relay to trip two circuit breakers? Is it possible someone was concerned that this LO relay was now a single point of failure so if that failed only one breaker would trip but there would not be contact closure to trip the other circuit breaker? Are typically 2 LO relays used for coordinated trip between two feeders to one source?

Thanks for the feedback!

Brian

 

[gar]

190725-1535 EDT

enterprisenx:

Your title to this thread has me mystified.

What took you in the direction that the "blocking diode" function was to block something (in a sense it does decouple, but blocking is not the diode's primary function)? Without the diodes the circuit performs the same gross function, but the relays 52 do not trip as quickly on relay 86 being asserted.

Then why "bypassing 86 lockout relay"? The diodes do not bypass 86 but rather they send the status of the 86 coil somewhere else.

SG-1 has confirmed my guess that the purpose of the diodes is to speed up the opening of the relays 52. This same objective could be provided by a fast acting relay whose coil was in parallel 86's coil. Could be a solid-state relay.

It appears that if relay 86 trip is energized, then both 52 relays are to be simultaneously energized to trip.

And if 86 is not energized to trip, then only one of the two 52 relays will be energized to trip if something else tells 52 to trip.

Clearly the logical "OR" function from several inputs is being performed to the 52 relays.

In the relay or sensor contact logic feeding the 52 relays there are several inputs being "OR"ed, all the contacts at the top left. Also there are two paths in that group that are an "AND" function.

.

 

[SG-1]

The diodes do speed up the trip, but they "block" the under voltage relay (27) from picking up the lockout relay. The lockout is used for a protective trip & disables the transfer . If the lockout has picked up someone must physically reset it. The system looses it's Good Line Seeking Ability.

The under voltage relay opens the effected Main breaker when the utility losses the line & then some other string of contacts not shown will close the same breaker when the voltage is restored to that line. The lockout must not operate.

In the medium voltage world we generally call diodes used in this manner either "blocking" or "steering". Blocking a signal or steering a signal to the destination.

Without those diodes there would be no automatic transfer, an under voltage on one line would trip & lockout both breakers.

Lockout relays are commonly used to trip multiple breakers including an entire bus.

 

[gar]

190725-1714 EDT

enterprisenx:

To get to the essence of your question draw a circuit as follows:

1. A +48 V DC supply line, and its associated common line.

2. Connect from +48 to a normally open switch S1.

3. From other side of S1 connect to a relay coil 86.

4. From the other side of coil 86 connect to DC common.

5. Create a second similar circuit with a switch S2 and a coil 52.

6. Connect a normally open relay contact from relay 86 in parallel with S2 forming an "OR" circuit.

7. Close switch S1. Coil 86 is energized. Three cycles later relay 86's contact closes, and then energizes coil 52. Thus, after 3 cycles both coils are energized by simply closing S1. If S1 is open, then closing S2 will not energize coil 86.

8. New circuit same as above, but add a diode from coil 86 to coil 52 so that when 86 is energized current immediately flows to coil 52 energizing it. The purpose is to energize both coils simultaneously.

9. Still if S1 is open, then closing S2 will not energize coil 86.

It is my opinion that calling the diode a blocking diode confuses its real purpose. The primary purpose of the diode is not to block something, but rather to shorten the time to energize a relay coil.

.

 

[SG-1]

The diode alternate purpose is to "block" the 27 under voltage relay from energizing the 86 lockout relay, as this would deactivate the automatic transfer.

The circuit could have just had a conductor connecting the middle to the two sides, but an under voltage on either main would shut down everything.

Perhaps the 50 shown on the one line was added later & the diode would compliment the instantaneous trip.

 

[SG-1]

Gar it is not the OPs terminology to call it a blocking diode. If I were talking with one of our customer order engineers he would use the same terminology.

 

[SG-1]

Enterprisenx, Look at it this way.

The MV breaker probably has a 5 cycle interrupting time.
The lockout relay adds another 3 cycles to this for an interrupting time of 8 cycles once the trip signal is sent. The longer the clearing time the more damage.

Adding a conductor with no diode will decrease the fault interrupting time back to 5 cycles, but it creates new problems:

If an operator is trying to parallel the mains so as not to drop load, when he opens one of the mains via the Control Switch the lockout rolls & everything goes off line.

In automatic mode the mains have good line seeking. When the under voltage relay opens one main the lockout rolls & all farther automatic operations are locked out.

Now we introduce the diode into this circuit. It makes sure that the 52 TRIPPING CIRCUIT only trips the one breaker. The diode blocks this circuit from sending a trip signal into the TRIP BUS CIRCUIT in the middle.

Lockout relays are very reliable devices & are used very frequently on single breakers or in the case of bus differential protection they usually trip every breaker on the bus including the TIE Breaker.

 

[topgone]

190725-1535 EDT

enterprisenx:

Your title to this thread has me mystified.

What took you in the direction that the "blocking diode" function was to block something (in a sense it does decouple, but blocking is not the diode's primary function)? Without the diodes the circuit performs the same gross function, but the relays 52 do not trip as quickly on relay 86 being asserted.

Then why "bypassing 86 lockout relay"? The diodes do not bypass 86 but rather they send the status of the 86 coil somewhere else.

SG-1 has confirmed my guess that the purpose of the diodes is to speed up the opening of the relays 52. This same objective could be provided by a fast acting relay whose coil was in parallel 86's coil. Could be a solid-state relay.

It appears that if relay 86 trip is energized, then both 52 relays are to be simultaneously energized to trip.

And if 86 is not energized to trip, then only one of the two 52 relays will be energized to trip if something else tells 52 to trip.

Clearly the logical "OR" function from several inputs is being performed to the 52 relays.

In the relay or sensor contact logic feeding the 52 relays there are several inputs being "OR"ed, all the contacts at the top left. Also there are two paths in that group that are an "AND" function.

.

That's how see it too: signal to trip 86 relay will also trip the left and right side 52 breakers!

 

[gar]

190725-1956 EDT

SG-1:

From what you describe it is apparently common usage to say blocking diode. But the real purpose is not to block something. The term blocking diode implies a different function than why the diode is used.

Without diodes the circuit does what it is designed to do, just not as fast. Replacing the diodes with wires changes the circuit function, and it no longer performs its intended function.

In my opinion whoever chose the term blocking diode made a big terminology mistake. Thus, I believe Enterprisenx was misled about the diode function.

.

 

[enterprisenx]

I really appreciate the discussion here.

This diode connection was introduced years ago and was not well documented and the engineer who would have been around at that time long retired.

Inside the cubicle door, it was labeled "blocking diodes" which is why I am referring to them as such. It is clear this wiring connection between the two feeders along the diode was added later (not factory wiring).

I also agree, that if it was just wiring added between the two trip circuits and no directional (aka diode) component, the undervoltage would cause a complete isolation of a bus where you really just want one feeder to be taken out. The only time you want both to be taken out would be on overcurrent trip. Again, I surmise that someone added this connection either that they didn't want to rely on the 86LO contact to close to take out the other breaker (time concern perhaps) or maybe they had issues with the LO relay and wanted to assure complete bus trip / isolation regardless of whether the 86LO relay worked or not. There is also the issue of maybe the LO relay was considered a single point of failure so this wiring could have been a mitigation.

So I was trying to figure if that is something I should leave in the circuit or perhaps I should just have it removed since there justification for having was not documented. I don't see anything wrong keeping it in but that's why I wanted to ask here to see if this has been done anyway and if I am carrying more or less risk having it in there.

 

[enterprisenx]

That's how see it too: signal to trip 86 relay will also trip the left and right side 52 breakers!

It's bypassing the 86 relay in the sense that the 86 coil and contact do not actually have to energize. Basically, if you take the 86 relay out of the circuit with the diodes, the overcurrent relay will cause a trip to both breakers. Basically, my understanding is prior to the diode wiring, an overcurrent trip one one feeder will only cause one feeder to drop out until the 86 coil was energized and thus causing the other feeder to drop out when the 86 contact closed. So yes it would now speed up tripping both breakers, but essentially you don't need the 86 relay in the circuit now to trip both breakers. Of course the 86 relay provides the manual LO function so with the contact in a closed state, you would still need to manually reset the 86 LO relay to close either CB.

Agree or disagree?

 

[SG-1]

Enterprisenx, check the rating of the diode, a 48VDC breaker may have a trip current of 15 - 20 amperes. We normally use a 40A 400PIV diode when it is in series with the trip coil.
If you do not mind I am curious if the breakers are Magnablast or DH ? The drawings indicate that they are antiques.

 

[enterprisenx]

Enterprisenx, check the rating of the diode, a 48VDC breaker may have a trip current of 15 - 20 amperes. We normally use a 40A 400PIV diode when it is in series with the trip coil.
If you do not mind I am curious if the breakers are Magnablast or DH ? The drawings indicate that they are antiques.

The MV breakers are GE Magneblast and relays are GE Type IAC.

 

[SG-1]

The MV breakers are GE Magneblast and relays are GE Type IAC.

Very cool, thank you & must be well maintained to still be in service.

 

[topgone]

It's bypassing the 86 relay in the sense that the 86 coil and contact do not actually have to energize. Basically, if you take the 86 relay out of the circuit with the diodes, the overcurrent relay will cause a trip to both breakers. Basically, my understanding is prior to the diode wiring, an overcurrent trip one one feeder will only cause one feeder to drop out until the 86 coil was energized and thus causing the other feeder to drop out when the 86 contact closed. So yes it would now speed up tripping both breakers, but essentially you don't need the 86 relay in the circuit now to trip both breakers. Of course the 86 relay provides the manual LO function so with the contact in a closed state, you would still need to manually reset the 86 LO relay to close either CB.

Agree or disagree?

The effect of bringing a trip signal to the breakers is to hasten the tripping of the breakers. When the protective sensors "relay" a trip signal to the respective breakers, the trip signal to the 86 relay is also routed to the same breakers, making a redundant trip signal.