overcurrent relay grading

The protection system relays are set in such a manner that the first relay in the feeder or connection to the fault occurence point detects and clears the fault, thereby avoiding tripping of the entire/ or other upsteream feeders. Accordingly, relays are time and current graded from the lowest power feeding point onwards and up to the highest system volatge distribution system.

Hence the relay in question is to be set like any other relay. Please note that a zig-zag transformer is mostly used to create an artifical grounding/ earthing system for a delta connected system The type of the grounding system provided does not have a bearing on the relay settings.

The book on protection relays by J Lewis Blackburn gives a good insight.

Hello veronica

The setting and adjustment of all protection relays is a job for the specialist Technician or Electrical Engineer, who has been properly trained and has the correct test equipment available to correctly adjust and set those protection relays.

Earth Faults can be extremely dangerous, even for persons and equipment at a considerable distance from the Earth Fault.

Discrimination Relays are normally set to ensure best safety for persons and equipment, by automatically disconnecting the least amount of equipment to satisfy safety for persons and equipment, both locally and remotely.

The Discrimination relays automatically send instructions or pulses to downstream and upstream Protection relays to disconnect whatever is needed.

Refer here: http://en.wikipedia.org/wiki/Relay#Protective_relay

These days, with very complex Power Networks, it is often not possible to easily calculate all the ramifications of a combination of protection relay settings.

The above has meant that in many locations, a computer monitors a full network through a series of subsidiary computers, and Software manages the complete Network, from Power stations through transformers, transmission lines, substations, local networks, and more.

What this means is that such a Software system may be vulnerable to "hacking" by nasty-minded folks, and in many cases the coupling between the various computers on that Network is not done via the Internet, but dedicated cables and/or microwave or fiber-optic links, which are more difficult to access.

The main aim is to protect persons and equipment, without causing a Cascade Fault.

A Cascade Fault occurs when protection relays and equipment are not set correctly for the particular electrical Network which is connected together, and equipment is automatically disconnected when it should not have been.

It is important when an Electrical Fault of any kind occurs, to ensure safety of persons and equipment (in that order), and thus the very minimum is safely disconnected to comply with that vital safety rule.

Read about a historical Cascade Fault here: Northeast Blackout of 2003

Trust that assists you, and if my reply is insufficient for you, please reply with

Kind Regards....