Neutral Grounding

Hello nileshkonduskar,

Not all electrical systems use the low resistance in the Neutral to ground connection.

The reason is mostly to prevent extremely high fault current in transformers, and alternators.

It is also so that a sensor may be connected across the resistor, and the ground current easily monitored.

Please have a read here:

http://postglover.com/lowresistance.html

Kind Regards....

Hello nileshkonduskar,

It is necessary for the designer to calculate (or measure) the earth fault loop impedance and make sure that it does not exceed the maximum specified in the wiring regulations. Having low grounding neutral resistance results in a small earth fault loop impedance and circuit can be protected, during faults, within a limited time.

For a radial system:

The earth fault loop impedance = resistance of the phase conductor+ resistance of the protective conductor + resistance of the grounding neutral

regards,

AMSG

Generally the relays are tripped on fault current and will be sensitive if the ressitsnace from neutral ground is low an dhence current high. So more sensitive to neutral voltage and trips the circuit.

Only used when you have tripping system on earth falut/ overcurrent etc. To ensure safety.

The loop resistance should take into consideration while calculating grounding resistance. But practically its bit difficult to calculate the value, since datas are not available at the initial stage of the project. Hence, the NGR is considered is the only restricting element for ground fault. Practically the actual earthfault current is little less than the calculated current. If the fault is not Bolted the current may further reduce. Its a general practice to consider only NGR value even at later stage of project.

This is an "off the wall" response.

Are you sure it's a resistor not an inductor?

I don't recall seeing an inductor in that application.

The grounding resistors, I have seen a few pictures where parts of them have been destroyed due to extreme fault currents or a bad mounting location resulting in corrosive type damage, some of these resistors are quite old, usually on low shaft speed hydro generators, can't recall seeing them at shaft speeds above 1200rpm.

The protective relay used measures the voltage across the neutral grounding resistor.

Hello CoronaCameraMan,

Inductors in a ground connection are sometimes used at AC to DC conversion stations, where Earth return circuitry is used - generally that type of DC supply is Volts DC (+) to Earth, and Volts DC (-) to Earth, with a common Earth return..

In that situation they are used for monitoring harmonics in the DC circuit Power output through the Earth conductor, and also via feedback loops and controllers, for reducing the harmonics present in the DC output from the AC to DC conversion station.

Kind Regards....

Hi,

Your post is not clear, so I'll give you another answer. Some electronic instruments have a resistor in the Earth connection (where the green wire goes). It is to reduce the effect of "ground loops", which can cause measurement errors. I assume it's low value (≈ 1 ohm) is keep some measure of safety.

S

There are also a number of code issues involved.

In some countries (Canada) there is a trend to solidly ground systems.

However, the first ground fault trips the breakers (that was the idea, right?).

So, you don't want your plant to go down on the first ground fault and you can tolerate a corner grounded system. But that is a fault condition that you must alarm, so you can repair it on a scheduled down day.

Now, for the 10 amp grounding resistors, it is not excessive power for a fault detection system and gives a reasonably stiff neutral connection to ground. This is good, because when you energize the high voltage side of the transformer you want to discharge the capacitive coupling between primary and secondary as quickly as possible. Also, the 10 amp grounding helps absorb transient voltages from noisy machinery and devices keeping the neutral close to ground in normal operation.

And then there is always the tradition. You can't fight tradition, without being declared a heretic!

Hi Everybody,

Wish you happy new year...

The function of the neutral earthing resistor is to lower the ground current to a certain value, accordingly, protecting the transformer against failure in the insulation level that might occurr as a result of sever ground current.