Ngr

[buwaleed99]

hi every one

direct to my equstion

I saw some transformers with Neutral Grounding Resistors (NGR) and other with out

caqn any one till me why some have it and others are not

Please i need the answer it is an important issue

 

[ibew441dc]

hi every one

direct to my equstion

I saw some transformers with Neutral Grounding Resistors (NGR) and other with out

caqn any one till me why some have it and others are not

Please i need the answer it is an important issue

250.20 lists Alternating Current Systems to Be Grounded.
(You are describing the following)
250.20(E) Impedance Grounded Neutral Systems. Impedance grounded neutral systems shall be grounded in accordance with 250.36 or 250.186.

250.36 High-Impedance Grounded Neutral Systems.
High-impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current to a low value.....

As stated these High Impedance Grounded Neutral Systems limit ground-fault current to a low value.There are very important restriction with the use of this type of system depending on the voltage. (see 250.36 or 250.186 depending on voltage)

The others could be solidly grounded or ungrounded. I am going to assume you are referring to the others being solidly grounded, in which the neutral is bonded to the EGC and the GEC without a resistor. In this case there would be a low impedance ground fault current path that would not limit fault current.

So why have one and not the other........Solidly Grounded and the circuit breaker can trip immediatly. High Impedance and the system alarms one to the fault so that the problem can be assesed, It may create a bigger problem if the breaker trips immediatly.

Hope this helped and Welcome to the forum!!

 

[buwaleed99]

Thanks it was very useful information

However

Would we assume that avoiding installation of the NGR in those transformer is to provide the adequate current to operate the protection relays when earth fault occur

 

[Pierre C Belarge]

The transformers without the NGR are supplying standard loads such as ligthing, receptacles, etc...


The transformers with the NGR are supplying loads that cannot be interrupted when the first fault occurs, as it may create a greater hazard upon opening the overcurrent device.

 

[don_resqcapt19]

The transformers with the NGR are supplying loads that cannot be interrupted when the first fault occurs, as it may create a greater hazard upon opening the overcurrent device.

Resistance grounded systems are often used in manufacturing processes where an outage is not a hazard...just a loss of production and dollars.