circuit overload protection vs overcurrent
circuit overload protection vs overcurrent
(OP)
Can anyone out there help me to understand the difference between overload circuit protection and overcurrent circuit protection. To me both look the same.






RE: circuit overload protection vs overcurrent
RE: circuit overload protection vs overcurrent
Perhaps it would be easier if you could explain the context of your question. Where are the two terms used with inconsistent meanings?
RE: circuit overload protection vs overcurrent
Overload is load current, not fault current.
In some cases, like general low voltage branch circuits, overload protection and fault current protection are both provided by the circuit breaker. Motors generally have separate overload protection.
The overcurrent protection for medium voltage utility distribution circuits generally is designed for fault current protection, not for overload protection.
RE: circuit overload protection vs overcurrent
RE: circuit overload protection vs overcurrent
Equipment normally has a rated current carrying capacity. If this is exceeded then the equipment can start to heat and if left unchecked can cause insulation failure.
If the current is overloaded for long periods, the thermal state rises and once it reaches 100% the relay operates. If the current falls below the rating, the thermal state starts to gradually fall.
If you had a situation where the equipment was overloaded for extended periods, but not all the time, the thermal state would be rising most of the timem but then falling for short perios. However the relay would eventually trip once the termla state of the equipment approached it's limits.
Now, if you used an overcurrent relay for the same situation the relay would pick-up as the current rose above the rated value, but once the current fell back below the rated value the relay would reset, and would need to start again. The thermal limits of the equipment would have been reached well before the relay operates and damage will have occurred.
An overcurrent relay may clear an overload condition, but that is not the purpose of this protection. It is there to clear faults, rather than overloads.
I do hear people use the term overload protection for when overcurrent relays are used, but this is not correct.
RE: circuit overload protection vs overcurrent
RE: circuit overload protection vs overcurrent
1. IEEE Std 100-2000 "Dictionary...."
"Overcurrent protection is a form of protection(s) that operates when current exceeds a predetermined value."
It may result from overload, short circuit, or ground fault.
RE: circuit overload protection vs overcurrent
An overcurrent relay will usually also operate for earth faults, however you need to set an overcurrent element above full load, whereas an earth fault setting can be much lower than full load current.
The o/c and e/f elements would both see the same fault current, however the current measured as a multiple of the setting will be much greater for the e/f so you will get faster operation.
It is also quite handy where delta-star transformers are present. The HV overcurrent protection needs to be co-ordinated with the LV protection, the e/f protection will not see an LV fault, therefore the co-ordination is not required.
There are probably other reasons too that I can't think of right now !!
RE: circuit overload protection vs overcurrent
1. IEEE Std 100-2000 "Dictionary...."
"Overload protection is the effect of a device operative on excessive current, but not necessarily on short circuit, to cause and maintain the interruption of current flow to the device governed."
The most known overload protection is the motor overload relay protecting the motor against overload but not properly against overcurrent, i.e. short circuit.
RE: circuit overload protection vs overcurrent
RE: circuit overload protection vs overcurrent
Overcurrent protection isolates faulted equipment, typically in a few cycles.
Overload protection is to prevent catastrophic failure or thermal aging, resulting from operation that exceeds equipment rating. Isolation of the overloaded equipment typically occurs in minutes, not seconds and certainly not cycles.
Michael Sidiropoulos
RE: circuit overload protection vs overcurrent
RE: circuit overload protection vs overcurrent
Overcurrent might prevent an overload condition occuring, but it also might not. Overload definitely will.*
* - providing it is set correctly !!
RE: circuit overload protection vs overcurrent
Overload and Overcurrent should not be confused as the protection relays perform completely different functions. An overload relay operates in the hundreds to thousands of seconds range whereas an overcurrent relay operates in the one to ten second range.
Cheers, Coingy
RE: circuit overload protection vs overcurrent
Overcurrent = ANSI 50 = instantaneous overcurrent
Overload = ANSI 51 = ac time overcurrent
Those two devices are often incorporated into a single 50/51 combination device.
RE: circuit overload protection vs overcurrent
I disagree.
Device 51 is time delayed overcurrent.
I only have a copy of the IEEE Std C37.2 - 1996. In this document, overload is never mentioned in the description for device 51, only overcurrent.
I belive a device 49 is required if effective overload protection is required.
RE: circuit overload protection vs overcurrent
1. IEEE Std 141-1993 (Red Book)
ANSI Device No. 50 Instantaneous Overcurrent Relay functions instantaneously on an excessive value of current (i.e. short).
ANSI Device No. 51 AC Time Overcurrent Relay functions when the ac input current exceeds a predetermined value, and in which the input current and operating time are inversely related through a substantial portion of the performance range.
ANSI Device No. 49 Machine or Transformer Thermal Relay functions when the temperature of a machine armature or other load-carrying winding or element of a machine or power transformer exceeds a predetermined time.
Devices 49 and 51 are somewhat related since they are substantially slower than Device 50. 49 tends to be linked to the low voltage motor overload and 51 tends to be linked to the medium and higher voltages current overload protection of motors as well as other loads.