Load Resistance of NGT

It is <...necessary...> so as to provide a measure of safety to users in and around the equipment and appliances connected to the secondary windings. Its <...resistance...> is there so as to limit the fault current to <...600 amps continuous rating...1680 amps 60 sec rating...> while giving the circuit protective equipment an opportunity to disconnect the fault, in this case a <...failure of connected cable...>, from the supply. For (a) reason(s) not known here, the protection equipment has failed to operate in time, which is why the <...resistance...burnt...>.

While the cable is being repaired, replacement of the resistance with an equal spare, and adjusting and verifying the correct setting of (all) the circuit protective device(s), are the additional actions to be taken. Do not re-energise the cable without having completed this beforehand, as lives may be under threat otherwise.

Thank you for your reply. in another instances. even load resistance is grounded .. then also resistance had burnt and system was not tripped due failure protection system. I feel the cause for failure of load resistance is not due to not being earthed. Failure of load resistance is not due to grounding and it is grounded. will it affect the protection system?

The forum has no information on <...another instances...>. So, in this instance, one does not need to bother with feelings or opinions; one needs to do what is in #1⇑.

Information that prevents a proper diagnosis is missing:

1) What is the vector group of the transformer?

2) What type of VDG relay, 11, 12, 13, 14, etc.?

3) What are the settings on the VDG?

4) Where is the cable that failed connected, to the primary, secondary, NGT; also is it shielded, how did it fail, and what is it length?

5) Is the main transformer and/or NGT frame grounded,

6) What are the grounding conditions, mat, station ground, ground rods, etc.?

7) What exactly does "burnt" mean, the element melted, arced; and where, across the turns, elements, or to the frame?

To your original question, grounding the element is a function of your protective relaying scheme, as well as the answer to the above questions.

Two instances of resistor failure suggest a problem in sizing of NGT/NGR. Probably, the current is greater than the rating of NGT/NGR.

The other possibility is problem in protection and fault clearance system. The fault clearance time is more than rated withstand time of NGR.

<...Two instances of resistor failure suggest...> that the Original Poster has a head-in-the-sand approach to flushing out the true cause of the problem...

For 6.6 Kv class what is your recommendation for tripping time.maximum voltage setting at VDG relay is 20 volt.NGT secondary voltage is designed for 400 volt.relay is communicated through 400/120 transformer.

The pickup and time settings form part of protection coordination study. Incomer protections you will not decide without finalising the settings for the outgoing feeders.

What can be said in general terms is that any fault is undesirable and needs to be isolated in the least possible amount of time, taking in to consideration the selectivity requirements of the system.