Multifunction relay vs breaker trip units

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OBI213

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What advantages do you get if you activate the overcurrent portions 50/51 on a multifunction relay when you already have trip units in the breaker?

Should the settings be the same on the protection relays as the trip units?

It is my understanding that multifunction relays are used for throwover schemes and that would be my reason to use them in leiu of discreet relays. However when it comes to selective coordination I am not sure if they should be the same trip settings or if they are even required as there is already protection in the form of trip units in the breaker.

Can anyone provide insight on this?
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
... or if they are even required as there is already protection in the form of trip units in the breaker.
There is no requirement for you to have (2) layers of overcurrent protection for a single device.
 

OBI213

Member
There is no requirement for you to have (2) layers of overcurrent protection for a single device.

Yes but what is good design practice? Just rely on the breaker trip units for protection and use the multifunction relays for the control scheme and not activate the overcurrent functions?
 

SG-1

Senior Member
Your Incoming Line Breakers could benefit from a multifunction relay programmed for 27, 59, 47, 67, & 32 functions. You can use the 51/50 device in the breaker for back up by setting the time delay longer than the multifunction device.

Multifunction relays are not required if overcurrent protection is already present. Unless you just want to build some redundancy into the system. Electronic relays with power supplies are only going to last a few years. If they do not have " relay healthy " contacts that you are monitoring, they could go offline and you will be unprotected, unless they open the breaker upon failure.

My eyes see a lot of switchgear & I can say that I have never seen anybody back up built in 51/50 relays with an external device.

Does this system have multifunction relays that are used for a transfer scheme ? That could cause coordination issues.
 
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jim dungar

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Location
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PE (Retired) - Power Systems
Be careful of using external Overcurrent relays to trip breakers that contain internal protection. The external relay will be controlling a shunt trip coil, so you will need to include its operating time when you are creating your trip curve. Not a deal breaker but something you need to consider.
 

jdsmith

Senior Member
Location
Ohio
Electronic relays with power supplies are only going to last a few years. If they do not have " relay healthy " contacts that you are monitoring, they could go offline and you will be unprotected, unless they open the breaker upon failure.

It depends on what brand of relays you buy and the type of terminations that are used on the back of the relay. Schweitzer has far better quality than other manufacturers in my opinion, and they have a warrantly to back it up. Experience in my plant has shown SEL's failure rate to be much better than their competitors as well - we have several hundred microprocessor relays in the plant, with the first installed in 1992. There are a number of other relay manufacturers that make fine products and have thousands in service with happy customers, and there are a few manufacturers that I would suggest folks avoid. Any relay without a 'relay healthy' or CTA contact should be avoided, and I'm not sure if any of the reputable manufacturers make microprocessor relays without them.

My eyes see a lot of switchgear & I can say that I have never seen anybody back up built in 51/50 relays with an external device.

Does this system have multifunction relays that are used for a transfer scheme ? That could cause coordination issues.

We have several main-tie-main 480V lineups from the late 60's and the early 80's that have electromechanical 50, 51, and 27 relays on the mains to provide inputs to the autothrowover scheme. Our newer 480V main-tie-main gear uses microprocessor 50, 51, and 27 relays to provide inputs to the autothrowover scheme. The mains also have trip units. For the autothrowover scheme to function properly and not transfer into a fault it needs to be assured that the external 50 and 51 devices pickup - the best way to do this is to coordinate them with the trip units on the mains. The primary 51 setting for the mains is in the external relay, and the trip unit is set slower than the external and coordinates with the external. The internal trip unit isn't required for protection in this scenario but since it's already there it might as well be enabled as a backup.
 

SG-1

Senior Member
We have several main-tie-main 480V lineups from the late 60's and the early 80's that have electromechanical 50, 51, and 27 relays on the mains to provide inputs to the autothrowover scheme. Our newer 480V main-tie-main gear uses microprocessor 50, 51, and 27 relays to provide inputs to the autothrowover scheme. The mains also have trip units. For the autothrowover scheme to function properly and not transfer into a fault it needs to be assured that the external 50 and 51 devices pickup - the best way to do this is to coordinate them with the trip units on the mains. The primary 51 setting for the mains is in the external relay, and the trip unit is set slower than the external and coordinates with the external. The internal trip unit isn't required for protection in this scenario but since it's already there it might as well be enabled as a backup.

I should have designated medium voltage switchgear. Built in relays for MV have only been around for a few years. The type I am familiar with has a mechanical contact that can be used to trip a lockout relay or provide an input signal to a PLC & thus stop the transfer. So far, I do not recall any lineups that had both.

I agree SEL makes a fine relay. Too bad you cannot get those built in. If you want longevity electro-mechanical is still the way to go. Tested some type ABB CO this past week.
 
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jdsmith

Senior Member
Location
Ohio
I should have designated medium voltage switchgear. Built in relays for MV have only been around for a few years.

What do you mean by built in? MV relays that are integral with the breaker instead of being mounted on the switchgear with separate CTs mounted in the switchgear? If the relay is built into the breaker like a traditional LV trip unit, presumably the CT/sensor is on the breaker as well?
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Nex Generation

Nex Generation

The VCP-T looks a lot like a Magnum DS with that on board trip unit.

Yes, looks like this new generation VCP-W MV breaker uses the LV Magnum operator to switch the contacts of vacuum bottles.
Personally, I think the design is a big mistake as I have experienced many problems with the Magnum design. One switchboard we could not shut down and the spring charge mech jammed. We could not open the breaker contacts to rack it out on the cell. We ended up
breaking the plastic spring charge handle to remove the cover, un jam
the ratchet to rack it out.
Also, lots of shunt trip and close coils burning up:roll:
Just bring back the old VCP-W breakers :cool:
Tony
 
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