Tips for a better Low Voltage Protection Discrimination Study 

Carrying out a protection system discrimination study is critical to ensure the correct functioning of  the electrical system in the event of faults.  The IEC defines discrimination as:

over-current discrimination

co-ordination of the operating characteristics of two or more over-current protective devices such that, on the incidence of over-currents within stated limits, the device intended to operate within these limits does so, while the other(s) does (do) not

Note – Distinction is made between series discrimination, involving different over-current protective devices passing substantially the same over-current, and network discrimination involving identical protective devices passing different proportions of the over-current.

Discrimination Curves
Image Source: Amtech Power

Electrical engineers are called upon to produce a discrimination study at various stages within a project.  If you are looking to have a protection discrimination study carried out or have been tasked with doing one, here are a few tips to help ensure you have a better than average study:

1. Do IT Early -
   
2. The earlier in the project a study is carried out, the better.  Ideally the initial study should be carried out in the design phase and then refined during construction and as equipment selection becomes finalized.   Leaving the study till the end of the installation, will make any necessary changes more costly to implement.  Carrying out a study earlier, will not only lead to a better-designed system, but can influence equipment selection and drive the cost of the system down.
3. Short Circuit and Earth Faults -
   Ensure the study covers both short circuit and earth faults.  In practice, both the short circuit and earth fault currents can be different and could affect the overall functioning of the protection systems.  In addition, relay and protective devices often have different settings for short circuit and earth faults.
4. More Than Curves [Current Limiting Breakers] -
   Often discrimination studies finish once the curves have been drawn.  IEC 60947 is clear that for current limiting breakers and high levels of short circuit, discrimination can only be verified by tests.  All manufacturers have carried out these tests, and reviewing their literature to ensure discrimination in this region in addition to producing discrimination curves for lower fault levels makes for a complete study.
5. No Source and Load End Discrimination -
   In complex systems, achieving full discrimination becomes almost impossible.  Avoid trying to discriminate between the source and load end of cables (if both have protective devices).  Both devices isolate the same section of the electrical system, making no difference if either one trips.  Any potential benefits of trying to narrow the fault location to within the cable is going to be lost in more time required for upstream devices to trip and longer fault durations.
6. Not Everything Needs To Discriminate -
   There is no rule that everything needs to discriminate and often it is impossible or costly to try and achieve this.  By understanding the system, it's operation and the effects on the business of power loss, a more engineered discrimination scheme can be achieved.
7. Use Software -
   In all but the simplest instances, you should use software to carry out both the fault calculations and setting of the protective devices (discrimination curves).  Trying to do things by hand, will lead to mistakes and compromise in simplifying the amount of work.  Software also allows the testing of what if type scenarios to improve system design, installation, and overall cost.
8. A Better Report -
   The result of any study is usually the protection discrimination report.  Spend time and effort on this.  Consider the reader and focus the report on them.  Keep it simple and avoid the temptation to expand it with waffle.  Identity why the study is being carried out, any major issues and the overall outcome.  Keeps settings, curves, supporting data, etc. in appendices.
9. Fully Document The Study -
   Fully document the study in a report.  Include only appropriate information in the main body and use the appendices for fully documenting the study.  Include all necessary information - supply authority technical information, short circuit and earth fault calculations, discrimination curves and current limiting checks, recommend settings and/or setting changes, problems and required system modifications/changes and any other technical information which is required as part of the study scope.

Hopefully, the above tips should help in achieving a better protection discrimination study.  The tips are generic and apply to nearly all discrimination studies.  However, bear in mind that each study is different, protection systems can be complex, focused around a particular technology or with a specific end user requirement in mind.  It is up to the electrical engineer to bring everything together and to deliver to the client a protection discrimination study which delivers results.