LV Breaker "Current Sensors"

[ZeroSeq]

Hi everyone,

I have searched this topic far and wide and have been unfortunately unable to acquire it, even directly from the manufacturer:

Does anyone happen to know the technical specs (ratio, excitation, burden, voltage rating, etc.) of the "current sensors" which are used for the electronic trip units on low voltage power breakers or MCCBs?

I know that they take a voltage signal from the sensors, and some of the secondary current is used to power the trip unit itself (I've seen between 18% up to 35% load current required to power the unit). However, that is as far as I can get. With respect to any other info on the "current sensor" / fake current transformer, I'm at a loss.


Thanks.

 

[Zogzog]

Well they are all different, have a specific one in mind?

First one I looked at (ABB MB) has ratios of xxxx:2A 600V 10kV BIL 1.06 ohm burden @12x

 

[ZeroSeq]

Can you tell me where you were able to find this info?

I'm looking for info on commonly used breakers: Magnum DS, Masterpact NW, Sentron WL, etc.


Thank you

 

[Zogzog]

Factory specifications for individual parts, the OEM's have that info, just don't want to give it to you. Often it is printed right on the CT though.

 

[mls1]

One note, the ABB MB used CTs for current sensors which is what most of the older breakers used. That was really convenient because they installed CTs with multiple taps so you need only change the plug to change the sensor rating. New breakers (for example Magnum DS) use Rogowski coils for current sensors. Rogowski coils are air core and produce a voltage output proportional to current, not a current output (the principle is the same but they don't saturate and they can't produce much voltage). The ratings of these two different types of sensors will be very different. Are you looking for ratings to be able to test them or to be able to use them in some other application?

 

[MatthewDB]

New electronic trip breakers usually include two sensors. For powering the trip unit electronics, they have iron core current transformers. The current measurements are made with rogowski coils. Rogowski coils are used because they remain accurate at overload and in the presence of harmonics.

The instant trip function remains magnetic so the trip unit can operate from a no load initial condition. From an initial no load condition the inverse time function can have a slight delay as the trip unit "boots" from its unpowered state.

 

[ZeroSeq]

Yes, that's one of my concerns: the ability of low voltage breakers to accurately sense a short circuit in the instantaneous region (zero delay or above the override) and to perform in accordance with the published time-current curves. I'm unaware of the level of saturation and delay for trip unit startup under heavy fault conditions. Just the concept of sensing only occuring after fault initiation is odd to me. It seems that given present relay technology, that this would be a design of the past.

 

[jraef]

Just a note of clarity, all of the breakers you referenced are Power Circuit Breakers (aka Air Circuit Breakers), you asked about Molded Case Circuit Breakers. Different animals in terms of construction and sensing.


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