Dear All,
Is it required to have separate earth buses for "protection earth" and "instrument earth"? I know that the earth grid is the same for both. But as per IEEE 1050, we have two separate earth bus. As per IEC 61000-5-2 all earth busbars shall be connected to each other in each storey of a...
Hi everybody,
Assume that there are two earth pits. Resistance of each of them is 20 Ohms (Calculated and measured).
We connect an Ohmmeter to these two pits. Assuming negligible resistance for wires which connect Ohmmeter to pits, what will be measured value?
Does it depend on distance...
dear all,
We have three types of earthing systems in our project: protection (power) earth, instrument (clean) earth, and lightning protection earth.
Usually we provide separate earth pits for each type. Earth pits for PE are connected to eaxh other, but not connected to earth pits of clean...
Dear All,
The IEC60909 is usually used for short circuit calculations in industrial premises.
It is applicable for meshed and unmeshed networks; for 3 phase as well as unsymmetrical faults; for far or near to generator faults.
It seems that we can use it everywhere.
But there is another...
Hi everybody
No-load & full-load currents of a new motor are too high. What can be the reason?
Motor is: 7.5kW, 50Hz, 4 pole, full load current as per nameplate is 15A, but it is actually 24A. Measured no-load current is 10A. Motor is new. I have checked the terminal box and certainly the power...
Dear All,
Many digital relays accept 0 to 1 A in their inputs for functions 50, 51, 37, etc.
But what is maximum acceptable input value for a relay?
As you know, current transformers saturate in case of short circuit. So they limit input current of relays.
To select current transformer, we...
Dear All,
Power cables installed beside each other, can not tolerate as much current as a single piece of cable.
It is due to thermal effect of each cable on the others.
IEC 60364 and IEC 60502 have specified correction factors for such arrangements.
I am going to put two layers of cables...
Dear All,
I have a cable which it's bending radius is non-standard and too high.
So we cannot connect it to the motor.
An idea is to remove its external sheath and armour. So it should be possible to bend the cable more easily.
What do you think? Is this method technically acceptable?
The...
Dear All,
I am going to read status of protection system (fault / no fault) from a Schneider Sepam relay via Modbus.
Unfortunately I do not know which memory address should be read.
I am able to read analogue signals (voltage, power factor, etc) but status of faults cannot be found in...
Hi,
What does the "1FS10" mean on nameplate of a current transformer?
"FS10" should be it's Instrument Security Factor and means after 10*Ip, the CT will saturate.
But what is the "1" before "FS"?
I have seen 3FS5, 0.5FS10, etc.
Are they based on IEC60044?
Thanks all
Dear All,
I have seen the following sentence about available interlocks in ABB's Installation and maintenance instructions for UniSafe:
"Locking electromagnet on the apparatus truck to prevent racking-in/out without power."
It seems that UniSafe prevents a breaker's truck from racking, if...
There is some discussions about using function 51 (time delay overcurrent) and function 49 (machine thermal relay) for motors.
Is it required or reasonable to use both for a large motor?
I have seen some plants which use both together, but only for motors > 1MW.
Some others use only 49.
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)...
