7 min read
A short circuit happens on one machine, and the main breaker opens along with that machine's device; the whole plant stops. The production loss costs far more than the fault itself. This is a lack of discrimination, and it almost always comes from the calculation never being done at the panel design stage.
What is discrimination?
Discrimination means that when a fault occurs, only the protective device closest to it opens while those upstream stay closed. The affected area is then limited to the faulty feeder. It is achieved two ways: by current margin and by time margin.
- Current discrimination — the upstream device's magnetic threshold stays above the downstream fault current. It works on low-current feeders and falls short at high currents.
- Time discrimination — the upstream device opens with a delay, giving the downstream one the first chance. Used between main and sub-distribution.
- Zone selective interlocking (ZSI) — devices are linked by a signal line so the downstream one tells the upstream that it has seen the fault. The fastest and most reliable method, but it needs breakers with electronic trip units.
Why "put the bigger breaker upstream" is not enough
The most common assumption on site is that discrimination is assured as long as the upstream breaker has a higher rating. It is not. In the overload (thermal) region it usually works, but in a short circuit (magnetic region) the two trip curves overlap. The fault current passes both magnetic thresholds at the same moment, so both open.
The same problem exists on the residual current side
With a 300 mA device in the main panel and 30 mA in the sub-panel there is a current margin, but without a time margin both can open. The upstream device therefore has to be selective (type S, time-delayed). A type S device gives the downstream 30 mA unit time to open first.
When should the calculation be done?
While the components are being selected, not after the panel is built. If the wrong device was chosen where discrimination is required, the fix is to change it — which means changing the panel layout, the busbar connection and sometimes the cable size. A calculation at quotation stage is far cheaper than a change on site.
Where it matters most
- Continuous-process plants — a stoppage brings restart costs (food, chemicals, glass, plastics).
- Cold chain — a stopped compressor means lost product.
- Data centres and control rooms — an outage is a direct loss of business.
- Hospitals and public buildings — regulation requires separated circuits to stay independent.
Send your single-line diagram; we check breaking capacity and discrimination at quotation stage. Working with Schneider Electric, Siemens, Sigma Elektrik and Himel, we can offer several budget levels for the same discrimination requirement.
Frequently asked questions
What is needed for a discrimination study?
The single-line diagram, the prospective fault current at each point, and the make and model of the devices to be used. If the fault current is unknown it can be calculated from the transformer rating, its impedance voltage and the cable sizes and lengths. Any claim of discrimination made without this data is guesswork.
Can discrimination be achieved between different brands?
It can be, but it is hard to verify. Manufacturers publish discrimination tables for their own product families; no such table exists between brands, so the curves must be overlaid and checked one by one. At critical points, staying within one family is both safer and documentable.
What can be done if an existing panel has no discrimination?
First, identify where the curves overlap. The remedy is usually one of three: replace the upstream device with a time-delayed model, move the downstream device to a lower trip class (C instead of D), or move to a breaker with an electronic trip unit and set it. All three mean changing a device, so there is a cost.
Which discriminates better, fuses or circuit breakers?
Fuses naturally discriminate better at high fault currents, because their clearing times fall steeply with current and the curves overlap less. A circuit breaker's advantage is being reusable and allowing fast restoration after a fault. On critical main distribution, a breaker with an electronic trip unit gives both discrimination and ease of use.