Necessity of Lockout relay for motor protection 3

[farale]

Dear All,
In some plants, all motor temperature faults are sent to a lockout relay.
For example the "time overcurrent relay" (51) function is latched by lockout.

In my point of view, faults which are serious and permanent should be latched. Including: short circuit (50), earth leakage (50G), unbalanced current (46)...
These errors certainly will not be removed after a motor restart. Technicians should repair the equipment before restarting it.

But the function 51 is usually due to an overload, not the motor itself. Nothing can be repaied by electrical technicians.

Do you prefer to latch this function in your design?
Sincerely

 

[Powerengineer_84]

Having to reset the lockout relay after any fault (electrical or thermal) will keep the motor from restarting after the event if some auto start signal is applied. If nothing else it simplifies the trip circuit by having all trips on a common trip bus. Like everything in protection and control engineering there is a balance to be struck. If the motor drives a critical load (such as a fire pump) allowing it to run or restart after certain faults may be preferable to locking out the motor. What load id the motor driving and where does it fit into the process?

 

[dpc]

Rarely have I seen an actual lockout relay on a motor. Much more common - trips from a digital relay are sealed-in until manually reset. For small motors with starters and motor overload relays, the motor overload relay is usually latched, requiring a manual reset. I'd prefer to latch the 51 trip. It should not be a routine occurrence and is an indication of a motor problem, arcing fault in the feeder, or problem with the driven equipment. A motor that is routinely tripping on overload is either undersized or there is some other problem.

 

[GroovyGuy]

Hi Falez,
I have never seen a physical lockout relay installed onto a MV motor starter. However I routinely install that functionality into a modern multi-function motor protective relay (MPR). (ie by requiring some one to walk up to the MPR and pressing the reset PB after a protection-trip). Is that what your install includes, all functionality in a single MPR (ie 27, 46, 49, 50, 51, 50G, 51G, etc)?
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[farale]

whether you implement the lockout via a physical device or use facilities of a microprocessor type relay to latch faults, it should be decided which functions need latch.

"Powerengineer84": I will follow your comment about critical loads (fire pumps). But I am not sure about other loads. For example a Boiler Feed Water pump is of high importance in the process. Should it be treated like a fire pump? Instead, I usually prefer to have an alarm for it on the HMI, before sending trip to the breaker. If something is wrong, operators have the chance to prevent a trip. But if the trip occurs, it is a good idea to latch
it.

"dpc": You are completely right about routinely tripped motors. Meanwhile I heard that inter-winding short circuits inside the motor may activate function 51, not 50. Then, the 51 is also an "electrical fault". Is that the case?

Thank you all.

 

[krisys]

To be precise, element 51 has no meaning here, when you have element 50 for over current/short circuit protection. You should set the element 49 (Thermal overload function)

 

[farale]

as far as I know, element 49 measures the temperature by a sensor (e.g. RTD) but 51 calculates it by checking current during elapsed time. (integrating I^2 * deltaT)
Am I wrong?

 

[oldfieldguy]

We have actual lockout relays on many of our larger motors. Generally, the multi-function motor protection relay will operate the lockout for short circuit, ground, and motor differential faults. In these cases, the operator is instructed to NOT reset the lockout until an electrical examination is made. Overload-related faults are assumed to be process-related, and are operator-resettable. The protection relay's thermal memory serves to prevent successive restarts on an overheated motor.

old field guy

 

[farale]

oldfieldguy: thank you for your reply.
Do you consider function 51 as process-related?

 

[dpc]

To be precise, element 51 has no meaning here

Going to disagree here. A 51 is a 51 and still has applicability. ANSI 49 element denotes motor overload protection, however it is provided. Not every relay has a "49" denoted function. A standard thermal magnetic circuit breaker can be used for motor protection, at least in the US. I wouldn't recommend it, however.

Latching the trips is the most prudent approach.

 

[krisys]

This is not to offend anyone here. Just to clear the air of confusion. Even if the motor protection is current based, the current is used to replicate the motor thermal characteristics.

Now a days in the era of numerical relays, the relay manufacturer can develop a precise equation to represent the motor thermal model based on the measured current. So ultimately this protection is falling under thermal overload category. Thus designated as protection element 49. Sometime to identify whether it is current based or thermal element sensing, there is a practice to denote 49 for current based and 49T for the thermal element sensing based protection.

51 is the inverse definite minimum time (IDMT) curve based protection element. It is used generally to achieve coordination with the upstream/downstream over current/earth fault relays in the power system.

During my early days one of my senior took sometime to explain this concept, when the Google guru was not available. Hence I clarified it here for the benefit of the members.

 

[farale]

Since discussion about 49 and 51 is not related to original purpose of this thread (lockout relay), I started a new thread about it.
Please participate there and let me and others to be well informed about this topic.
Thread subject: should I use both element 51 and element 49 for the same motor?