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outofcontrol (Electrical)
5 Feb 03 2:24
Hi
I am currently doing brownfields work on offshore platforms in the oil and gas industry. Can anyone explain why I continually come across the use of shunt trips in critical systems isolation circuits ( eg ESD trip of power ), when an undervoltage trip coil is fail safe, eg if the wire falls off or the coil burns out the breaker trips. With a shunt trip you don't know it won't work until too late. Is there a reason for this ?

In marine rules for shipbuilding, DNV, Lloyds etc do not allow the use of shunt trips for this reason.

Thanks
DanDel (Electrical)
5 Feb 03 8:30
You're comparing apples and oranges. Don't forget, protection is not limited to undervoltage. Shunt trips are very dependable and universally used for tripping, and can be activated by any relay, not just a non-adjustable voltage coil.
Perhaps for a specific application, where just U/V is required, and just for opening a C/B under specific U/V conditions, the U/V trip coil might be considered more dependable.  
jlazucena (Electrical)
5 Feb 03 10:01
When using “shunt trip” you can have the following options to know when the tripping circuit is healthy:
Indication lights in parallel with tripping contacts (The light is in series with the tripping coil). Note: this also applies to the closing coil and that is one reason why you have at least two colours to indicate breaker position.
Some digital relays have the features “BROKEN CONDUCTOR” (Please do not confuse with “breaker failure”)
You can even interlock high and low side of a transformer for example by using a “trip transfer by direct wiring scheme” (Two 85 relays -sender and receiver).

Some times you do not want to trip, but only to alarm when something is wrong in the tripping circuit.

Above all, there is not substitution for good: design, installation, commissioning, maintenance and operation.  You must know the type of installation that you have to operate and how to detect when something is wrong.
outofcontrol (Electrical)
5 Feb 03 19:18
Hi guys

Thanks for your replies, but maybe I did not explain myself so well. I am not talking about a trip on low system voltage, but the means of actually trippping the breaker from a shutdown system. The application is the use of tripping means on circuit breakers on an offshore gas platform - there are two commonly used possibilities

1) a shunt trip coil, which must be energised to trip

2) a undervoltage coil, which must be energised to allow breaker to close or remain closed.

These breakers may be called upon to trip in the event of, say for one example, gas detected on the platform, to remove power to the platform to remove potential ignition sources. To have a non-failsafe means like a shunt trip seems non-ideal, unless the loop is monitored, which these are not. The problem with shunt trips is that if not monitored, and a wire is disconnected in the trip circuit, you won't know until you try to trip and can't.

Everything else on a gas platform for safety shutdown is done in a fail safe manner eg maintained signal asserted high to allow run/operation of equipment. Why then go to a non failsafe arragement for the last part of the loop? Is there something I am missing ?

Thanks
DanDel (Electrical)
5 Feb 03 19:53
I understand you now, though I'm not sure of the voltage level you are concerned about. This type of fail-safe trip is not typically used for C/Bs of any voltage, though there are more backup trips and coordinated devices for MV(2kV-69kV) and above C/Bs in the event of a shunt trip coil that fails(a very rare occurence). I would expect in the long run that a shunt trip coil is more dependable than a constantly energized voltage coil. I've seen U/V coils burn up and melt without dropping out and tripping a C/B.
If you are this concerned about a fail-safe mechanism for a C/B trip, you'd be scared to death if you saw how a molded case C/B thermal-magnetic trip works(<600V).
Of course, you may be talking about some process control systems which require this fail-safe operation. In this case, you could use electromagnetically-held contactors opened by external relays along with the C/B.  
Also, for most industrial plants, the concern about fail-safe operation is not anywhere near the concern about nuisance tripping for no problem at all(or a burnt out U/V coil), which might shut down a continuing process causing large financial losses.
jraef (Electrical)
5 Feb 03 20:40
Another criterea for using Shunt instead of UV trips is the normal power operation of the system. If for instance power is shut down on a regular basis, i.e. end of shift, UV trips would trip the breaker and require resetting at every startup. Or even if power failures are just a common event, using a UV trip can quickly wear out the trip mechanism on a CB and cause possible failure in the event of a real fault. Beleive it or not, mechaical operations on CBs are listed in the thousands, not millions like contactors. Square D for instance lists an "endurance" rated breaker at 1000 operations loaded, 5000 unloaded. Once per day for 3 years is already over a thousand operations.

Quando Omni Flunkus Moritati

busbar (Electrical)
5 Feb 03 23:52

In medium- and high-voltage circuit breakers, operation by energized solenoid or "trip coil” is almost [if not completely] universal.  If reliability needs to be increased, then multiple trip coils may be specified.  In utility and industrial switchgear, they [and their “energize-to-trip” logic of operation] seem to have a widely accepted track record.  
  
Helpful Member!  bigamp (Electrical)
6 Feb 03 8:22
outofcontrol,

I guess you are refering to MCCB's mostly.  I think shunt trips have been around "forever" but I suspect UV releases may not have been around all that long.

In the electrical world, we traditionally energise to trip.  Our switchgear and the tripping supplies (traditionally something like 110V DC from batteries) are really reliable.  Traditionally, electromechanical protective relays would have normally open contacts that closed to initiate a trip.

Many shunt trip based control schemes impose no load on the DC tripping supply when in a healthy state so there is no worry about battery bank capacity and autonomy time during loss of AC conditions.  A UV based tripping system will always impose a load on the DC system.  This will be no problem provided these loadds are taken into account in sizing the DC system battery capacity (and autonomy time).   

In the instrumentation & control world, people like to be "fail safe" hence the use of UV coils which will typically operate at 24V DC and be controlled by things like the ESD system, fire & gas system, SSDS etc.

Possibly in the early days of platforms, when there were more electrical engineers around they would have used their preferred approach, which would be shunt trip.  As instrumentation engineers prolifirated, they would have started to insist on the use of deviant technology (UV release), IMO correctly for this application.

IEC type LV ACB's as well as MCCB's all seem to have a UV release option and some makes of MV switchgear are able to offer UV release as well.

Regards   

busbar (Electrical)
6 Feb 03 12:57

As implied by others, breaker tripping in some cases can be undesirable [id est, unsafe.]  
  
electricuwe (Electrical)
7 Feb 03 15:27

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