circuit overload protection vs overcurrent 3

[isaacs]

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

 

[jstickley]

In my opinion, it's a matter of interpretation. If you tell me a device has overload protection, it implies to me that there is a protective relay that will operate to open the device when it overloads. If you tell me that a device has overcurrect protection, it tells me the kind of protective relaying that it has, rather than the level at which that relaying will operate.

 

[DanDel]

isaacs, I would think that most people would agreew with you and use the two terms interchangeably. The only difference may be in the definitions of 'load' (power) and 'current' (amperage), but an 'overload' device doesn't actually react to power, it reacts to current.
Perhaps it would be easier if you could explain the context of your question. Where are the two terms used with inconsistent meanings?

 

[jghrist]

Overcurrent is any current in excess of equipment ratings and can be either load current or fault current.

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.

 

[isaacs]

Thanks to all you guys who have so far helped me understand the difference or the lack of it between overload and overcurrent protection. But i would also like to know why there is also an earth fault protection on HV lines in addition to o/c, yet an earth fault will cause excess current to flow.

 

[DiscoP]

Overload protection normally uses thermal replica to measure the thermal state and determine if an overload condition is occurring.

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.

 

[jnims]

Having seperate ground fault protection is a good idea because ground relays can be set more sensitively. For example, consider a distribution feeder with a maximum normal load of 600A. To prevent nuisance tripping, it is necessary to set the phase time overcurrent relays at 125% to 150% (or greater) of the maximum load current. A reasonable pickup for this circuit could be 800 to 900A. If this circuit is a 4-wire multi-ground circuit, you would expect the return current to be no greater than 20% of the maximum load current. If the neutral current is greater than 20%, this is probably a good circuit for a balancing project. We could set the ground time overcurrent relay to pickup at 160 to 180A. The ground relay will be better able to detect and trip for high-impedance ground faults. The disadvantage of a relay set the low is it can overreach large fuses. For faults above 3000 or 4000A, it probably does not make a great deal of difference depending on what you are trying to coordinate with downstream. On subtransmission systems (where there should be little or no return current), I have seen pickups as low as 80A with time current characteristics that let a downstream differential relay clear a substation fault. This allows for very fast ground fault protection.

 

[jbartos]

Suggestion: Reference:
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.

 

[DiscoP]

I agree with jnims.

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 !!

 

[jbartos]

Suggestion: Reference:
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.

 

[wired1]

Note that overload implies heat. Overcurrent implies magnetics. Two types of failure you are attempting to address in circuit protection. You have each without the other. Correspondingly, fuses or breakers can be designed to handle one or the other or both types of protection.

 

[SidiropoulosM]

Mine is a simpler understanding:

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

 

[jbartos]

Suggestion to the previous posting. The overcurrent protection will prevent more serious destruction than the overload protection. As you mentioned the overload protection can result in catastrophic failure, thermal runaway, etc. However, the overcurrent can shatter equipment into pieces because of electromagnetic forces. Then, there is a very little of equipment left in extremely short time. There may be very vast energy release in the very short time interval. Also, no one may even have a chance to run away. I would not recommend to experience it.

 

[DiscoP]

In simple terms- overload protection will not clear a short circuit in time. Overcurrent will.*

Overcurrent might prevent an overload condition occuring, but it also might not. Overload definitely will.*

* - providing it is set correctly !!

 

[coingy]

Isaacs,
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

 

[peebee]

To clear up some misconceptions in the first few posts regarding the interchangeability of the terms overcurrent and overload

Overcurrent = ANSI 50 = instantaneous overcurrent
Overload = ANSI 51 = ac time overcurrent

Those two devices are often incorporated into a single 50/51 combination device.

 

[DiscoP]

peebee
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.

 

[jbartos]

Suggestion: Reference:
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.