Sensors, plugs and Frames

[aqua1985]

I came across the concept of sensors, plugs and frames used in switchgear in the United states. I haven't been able to fully grasp the need and working of the above. Until now, i've only heard of current transformers used for wiring relays and metering. Could someone explain and give me some material to read up on the use of sensors, plugs and frames?

Thanks,
aqua

 

[DanDel]

These three terms make more sense when you consider that they were developed for older metal-frame power C/Bs.

A frame rating is the maximum current for which the frame (structure of the C/B) is rated.

The sensor rating is the CT rating (or coil rating of an older electromechanical style trip unit).

The plug rating is the current rating of the removable plug which is included in most microprocessor-type trip units. The plugs can have ratings up to the rating of the CTs. They are used to lower the pickup of the trip functions, limiting the capacity of the trip unit.

 

[Zogzog]

Say you have a 1600A frame breaker. You can put CT's on it from 1600A on down (1200A, 800A, 400A), the settings on your trip unit are multiples of the CT rating. But there is a limit to the adjustment range (Based on limitations of the CT's), usually down to 0.5x for the LT pickup. So if you have a 1600A breaker and CT's and a 400A you cannot protect it very well.

Now some trip units allow you to change the CT tap internally to the trip unit, others require you to either change the CT's, change the taps on the multiple tap CT's, or worst case change the rating plug to a smaller one so your trip settings can be adjusted in the correct range you need.

They used to use multiple tap CT's, problem with that (If you are the OEM) is that they are easy to change. enter the rating plug, now you need to buy a new part from that same OEM;s and rating plugs are not cheap. That is one reason I prefer fully programanle trip units, just change a setting and you are there, no extra parts to buy.

 

[dpc]

There are some advantages to the rating plug. NEC Article 240.6.B requires that for adjustable-trip devices, the breaker "rating" is considered the maximum possible setting, subject to some leeway given in 240.6.C for certain situations.

An 800 A sensor set at 0.5X (400A) pickup, would still have to have conductors sized for 800 A unless the installation met the requirements of 240.6.C. With a 400 A trip plug, that would not be the case, and 400 A could be used for sizing the conductors. I believe this was the main reason behind the use of rating plugs.

 

[Zogzog]

Same goes for changing a CT tap. Also, you can program the "rating plug" (That is what they call the setting) on some trip units and password protect it so it cant be changed easily by unauthorized persons. Same as a rating plug.

After that all your settings (LTPU, STPU, etc) are all based off of those ratings. Rating plugs are all about the money.