don't know significance of Let through Energy in Low Voltage Power system.

is that earth fault you are talking about?

or

The dynamic peak of the first wave has an electromechanical impact on busbars, cables, their supports, closed contacts of downstream devices, etc. In a well co-ordinated system, no mechanical damage should occur.

The other part I²t, has obviously a thermal effect. Again, in a well designed system, no permanent damage should occur.

The dynamic peak as well as I²t values of switchgear are essential for proper co-ordination in a distribution system.

Thanks a lot for that.

If we consider I²t effect wise then there are several application where it affects significantly, like Type 2 coordination for motor protection, design of busbars in Panel Boards.

1. In type 2 coordination, we use selection chart provided by manufacturers. But how they design it? Is I²t is significant factor while designing type 2 coordination chart? Because we will have to use 1 rating greater than what we require?

2. How does it will effect bus bar design and construction if we decrease the I²t value of all the switchgears significantly?

Thanks a lot. . ..

1. Type 2 coordination has to be established by actual testing. Some interpolation/extrapolation may be possible within one relay frame size, by testing the highest and lowest current settings of that frame size. In my experience, relays of >1 A and <16 A were more vulnerable to permanent damage during SC test especially with MCCBs as SCPD. Fuses are better.

2. Regarding busbars and supports, it is generally the dynamic peak that needs to be watched, since thermal damage is less likely.

While it may seem very good to have current-limiting devices (fuses and MCCBs) everywhere to keep I²t low, what about continuity of supply to healthy feeders in the event of fault? Non-current-limiting switchgear, ie, ACBs will be around for quite sometime more..

1. I apologize but I am not agree with you, I don't think so that each manufacturer will be testing all the combination ( DOL and Star Delta, using MCCB, Fuses, MPCBs and all) of type 2 coordination. They must be overdesigning the whole system of type 2 coordination such that no permanent damage occur to the downstream devices or contactor in type 2 system.

2. But sir, there are some Cat A ACBs that are available in market.

In present industry, all the downstream devices are generally overdesigned. like motors are totally overdesigned to withstand let through from upstream devices. so this increase copper usage in motor thus increasing total cost of the motor.

Why is it so?

1. The three MNCs i worked for did test everything. Remember, in Europe and USA, what you say in your catalogue is binding on you, and if there is a failure, you can be sued. So, they test everything in their labs. A big job, but nevertheless necessary. However, they do calculate before...the dynamic peak will make the contacts of the contactor repel, and it can weld when it closes again. So, they simulate and establish the repulsion threshold by a low voltage high current pulse. Similarly the failure point of the relay by passing a low voltage high current pulse, for a few milliseconds. OK, this was before the extended use of FEA packages, and it may be that such things are computer simulated.

2. ACBs mostly have Icw ratings of 1 sec, mainly for usage upstream of MCCBs. Those which do not have, will be only able to control one feeder or a small switchboard, not common.

Motors in India are overdesigned to take care of the high currents that they will draw on low voltage, which is very prevalent here. Not for let-through. Think about it...motors are final loads. No current flows beyond them is it not ?

Yes that can be one reason behind overdesigning motor. Any way in India every one uses the device which one rating higher than what is required!

But let through should be one the reason because to carry over current in low voltage is continuous duty but what about in case of short circuit it has to withstand the very high current for short period time.

Because I have seen on the name Plate of the Motor that they also specify the Intermitant Duty of the motor in Current in kA and in time?

So I think so they must be testing it for short circuit fault also!

So they must be over designing the motor for let through also.!!

Step back and think a bit....

How can you Short-circuit a motor ?

If it is designed to run in star, you have only 3 terminals. If you short them, the motor is completely out of the circuit isn't it? No current will flow through the windings of the motor at all ... yes?

If it has all six leads available outside (star-delta starting)...if you short the three ends, the motor is in star connection anyway, its full impedance is in the circuit, and it will run.

So, you cannot have a short-circuit current passing through the motor, unless there is a short in the winding itself. But then, in that case, the motor is gone anyway isn't it?

i am surprised that you saw a kA rating..maybe its starting current was in kA already (a very large motor)? And generally expected to be used intermittently eg, Cranes ?

If we consider I²t effect wise then there are several application where it affects significantly, like Type 2 coordination for motor protection, design of busbars in Panel Boards.

1. In type 2 coordination, we use selection chart provided by manufacturers. But how they design it? Is I²t is significant factor while designing type 2 coordination chart? Because we will have to use 1 rating greater than what we require?

2. How does it will effect bus bar design and construction if we decrease the I²t value of all the switchgears significantly?

Thanks a lot. . ..

Yet another job for British Standard 7671.... <sigh>