MV Breaker Trip Circuit

[JMatt]

Unless I am mistaken, typical trip circuits (125VDC) have a trip coil with a 52a contact on EACH side of the coil. Recently, I came across a design with a two 52a contacts in series on the positive side of the trip coil, and the trip coil itself was connected directly to the negative DC supply.

Can anyone provide some insight as to why the typical design has a TC flanked by 52a contacts? Should I be concerned that this design isn't "typical"?


In other words...
(+DC)--||--TC--||--(-DC)
VERSUS
(+DC)--||----||--TC--(-DC)

Any thoughts are greatly appreciated.

 

[slavag]

Hi.
I don't know ANSI/IEEE standard for MV trip coils, IEC standard is one 52a N.O. contact in series with TC."-" is connect directly to TC.
This 52a contact is disconnected trip signal, reason:
1. TC not energized more by trip contact or LOR (prevent damage of TC).
2. Trip contact have make and carry for 110/125V DC around 5/30/15A (cons/0.5sec/3sec) and breaking capacity <1-3A
(two trip contact in series connection). 52a contact is prevent damage of trip contacts.
Data of trip contact is averrage.
Regards.
Slava

 

[dpc]

Key concern is to break the current flowing through the trip coil once the breaker is open. The two contacts in series is generally to provide increased interrupting rating to break the dc current and probably some redundancy in case one contact welds shut.

There are some pros and cons of putting a contact on each side of the coil, but it's done both ways and I wouldn't worry about it and I wouldn't change it. Stick with manufacturer's design.

 

[oldfieldguy]

JMatt-

I just grabbed a drawing out of my car for a Siemens SF6 breaker and it has a single "a" contact in series with each of its two trip coils.

Off the top of my head, I seem to remember some old equipment having the coil isolated by an 'a' contact on each side. The former is more likely a modern convention.

The 'a' contact opens when the breaker trips to remove the current from the trip coil. Trip coils are not generally rated for continuous application of current and will burn up if the breaker does not open the 'a' contacts to remove the trip signal.

Since some tripping devices latch their output contacts closed until reset, the action of the breaker MUST interrupt the trip coil path.

old field guy

 

[RRaghunath]

Connecting the coils in electrical scheme (including the trip coil) directly to the DC system negative is the preferred practice currently.
This gives protection to the coil (the solder joints more specifically) against corrosion. The water is repelled by the negative charge on the coil.

 

[JMatt]

Thanks for the info, people. You've helped a lot.