should I use both element 51 and element 49 for the same motor? 2

[farale]

There is some discussions about using function 51 (time delay overcurrent) and function 49 (machine thermal relay) for motors.

Is it required or reasonable to use both for a large motor?

I have seen some plants which use both together, but only for motors > 1MW.

Some others use only 49.

 

[LionelHutz]

Both curves are supposed to be time-current curves so the curve which is "lower" will always trip first and make the other curve pointless to be programmed.

 

[JG2828]

Depending on the relay, the 49 function should take into account the cooling characteristics of the motor, where the 51 relay will not. So if the motor has been running at full load, or the motor has been recently started and stopped, the 49 element should trip faster than if the motor were cold.

 

[DiscoP]

In addition to JG2828's comments, with the relays I have used the 49 element also takes -ve sequence current into account, and speeds up the operating time when present.

 

[oldfieldguy]

Modern motor protection relays handle the '49' function very well, taking into account, if needed (and programmed properly) current imbalance, cooling times, etc. It is still called '49' but it exceeds the traditional application of the term.

The same can be said of the '51' function. In the days of discrete electromechanical relays installed for various functions, a time-overcurrent relay might be for many different functions, for instance as an incomplete sequence timer as well as other functions all contained within the little box of modern microprocessor-based devices.

The approach you probably need at this point is to decide which protection and supervision functions you need, and then to look at how your protective devices might provide them.

old field guy

 

[krisys]

In this era of numerical/microprocessor based relays, where the relays are considered to be providing the entire protections for one application in a package form. Feeder protection, motor protection, transformer protection package etc.

The ANSI device numbers are there for a long time and not revised. Hope with the fast changing landscape of the way the protection function works and its principles/theory, ANSI will come up with a revision for the existing device numbering system. Until then there is no other common platform to bring the protection community together and discuss at the same level.

I am out of the academics for a long time. Really curious to know how the relay protection subject in taught in the college these days.

 

[farale]

"I am out of the academics for a long time. Really curious to know how the relay protection subject in taught in the college these days."

I am also out of academic atmosphere for more than 19 years, but I guess relay & protection can be taught in university just based on theoretical method. There is not practical experience in the universities, about this subject.

 

[oldfieldguy]

krisys-

That's part of my point. For instance, GE implements a "50" function on its devices. ANSI 50 is an instantaneous overcurrent function, where 'instantaneous' means 'no intentional delay', but in GE's version, one may program an intentional delay for the element, which in my confused opinion turns it into an ANSI 51, definite time overcurrent.

That's why I say that one should understand which protections a motor should have an understand how to use a particular device to cover those functions, despite the manufacturer's free use of ANSI device numbers.

old field guy

 

[krisys]

oldfieldguy,

We always insist a legend in the Single Line Diagram (SLD, or in North America they call One Line Diagram). This legend is specifically listing the protection function ANSI number with the meaningful description against it.

49T Motor thermal overload protection, RTD input
49M Motor overload protection, current sensing
51LR Motor Locked Rotor Protection
51 IDMT over current protection
50 Instantaneous overcurrent protection(with or without adjustable time delay)
etc.

This gives an improved representation for easy understanding.