Earth fault tripping

Data that mey be required..

Is the inverse relay a inverse time or a inverse current relay ?

Sudh.

In order to better evaluate your problem, a lot more information is required. I am mainly familiar with US power systems and do not understand some of the terms used in other countries.

What is your system? Is it single or 3 phase and what is the configuration and how are your earth sensing elements connected.

In general, basic ground relays are only effective on three phase ungrounded Delta connected systems and operate on the principal that the summation of the instaineous 3 phase currents in the Delta circuit must equal zero, (unless there is a fault). 3 current transformers are required (one on each phase) and fed to an inverse time over current relay. If yours is an older disk type relay how many time levers is it set at.

Another method, If the system is fed as a Delta circuit from a Wye secondary connected feeder transformer. then a single Current transformer can be connected in the ground leg, but this method can only be used at the source.

As you discribe it, It sounds like your feeder breaker is an instaineous trip and the down stream breaker is a time trip. In this case the coordination is backwards. Your feeder breaker MUST be a delayed trip in order to give time for the down stream breaker to trip. The inverse time overcurrent function allows a fast trip only when the current magnitude is so high that damage will result.

As you see, a lot more information is needed to help you with your problem.

To my mind, Snakers has given you the best possible correct answer.

It would appear that the person who designed your system was on a "bad hair day" (ask any Lady if you do not know what is meant here!) when he designed it!!!

Change the characteristics round (if possible) or buy new breakers to ahieve just that....

The setting CBCT 50/5A set at 50mA means that the relay will pick up at a primary current of 50x10mA or 500mA. and it will take .5 sec to trip the breaker

The IDMT CT 2000/5A with EF set at 2A - 10 sec means that the relay will pick up at a primary current of 2x400 A or 800A with tripping time depending on the relay characteristic curve.

Therefore the relay settings are not adequate, unless you have made a mistake in reading the settings on the IDMT which normally has settings for both the overcurrent and earth faults. The 2A - 10 sec setting appears to me an overcurrent setting rather than an earth fault setting.

Is the IDMT a particular brand of solid state relay/ perhaps schumberger or the other big European manufacture that I cannot think the name of ? They were coming out with nomenclatures of this sort about the time I retired. I am used to the IEEE terminalogy.

Hello,

Thanks for your valuable information.

I have checked at IDMT the setting is same as 2A for 10 sec, as u mentioned it is correct relay settings are not adequate and not properly co-ordinated.

We have to find out solution for this kind of tripping as both breakers are having different characteristics.

Regards,

Sudh.

Dear Sir,

I feel youhave tochange the cuirrent setting from2A to4Atomatch the mother earth impedence if u do this your problem gets solved

Rgds

S.Purushotham

My Electrical friends,

This may be out of syllabus. But I must let you know. We also had earth leakage relays at our office but they miserably failed and we stopped using and replace it with a new one, because the Earth Leakage Relay would always be in tripped condition which was because of the more than moderate leakage current from the SMPS of PC's, servers and all other equipment that had SMPS. What we do nowadays, for many years, is once in a while check the potential between Neutral-Earth, Neutral-Phase, and Earth-Phase, < 2.0VAC, Supply voltage and Supply voltage-(<2.0VAC) respectively. Has anybody come across some device for this purpose to continously monitor and raise alarm on threshold crossver?

R. GANAPATHI RAO,

C-DOT,

BANGALORE, INDIA.

I have worked the last 26 years in offices where PCs were in abundance. We also had ELCBs for everything. The only time I ever saw one trip was when I personally accidently touched the live on an open power supply. It hurt, but did me no damage (other than to my pride as I was teaching a class of computer engineers at the time!!!)

In my experience, when some devices get slightly "leaky" (a kettle I once had SOMETIMES dropped the breaker when used, but not immediately and not always) and get used together, the problems happen.....

I see no reason (other than a fault or faults) for your problems, sorry......

The fix sounds as though it has removed a level of safety for you and your colleagues.......