This one is regarding faults in electrical systems.

Thread Starter


Joined Oct 18, 2006
Hello friends,
I would like to know the difference between overvoltages & overcurrents. Which is more dangerous & why do we normally use an Inverse definite minimum time(IDMT) relay for an Overcurrent relay & a definite time relay for an over-voltage relay. Is this because that in case of overvoltages, in spite of heavy voltage there wouldn't be too much of a damage or what's the actual funda in the whole thing.


Joined May 16, 2005
Over-current is more dangerous. Conductors are selected to dissipate a safe ammount of heat at operating currents. A bit of "overhead" is calculated in to insure the conductors never approach insulation-damaging temperatures. Over-current condition can very quickly overheat the conductors and ruin the insulation. Then bad things begin to happen. Over-current relays nip this scenario in the bud.

Over-voltage relays accomodate voltage spikes.

I don't know what "funda" means.


Joined Nov 21, 2006
I agree in principle with thingmaker. However, like in most other situations, the answer is "it depends". Over voltage in some circumstances is most surely a critical problem. As with generators and transfomers. When voltages get too high in these devices, (usually about 10% higher than normal) excitation current in the iron core becomes excessive and grave damage can occur quickly. (In generators, this is sometimes referred to as over fluxing)
On the other side of the coin,
if your protection goal is aimed at conductors, then overcurrent relays would probably be chosen as the primary protection system. As thingmaker stated, conductors and where they are located (overhead, underground, inside of enclosures or raceways) are limited in current carrying capability. To exceed that capability means damage from overheating, or outright "burndown" and destruction. Overvoltage and overcurrent relays can play equal roles, it just depends on the application.