SHALL ONE UPGRADE THE SWITCHBOARD IF MOMENTARILY DURING THE SWITCHING (MAKE BEFORE BREAK), THE PROSPECTIVE SC LEVEL EXCEEDS THE SC RATING OF THE BOARD?

A: Issue:

Persons Under Competent Control, Chargeman, Electrical Engineers and Electrical Service Engineers will always perform switching at electrical switchboards from LV to HV. With increase in fault level in the system (due to more generation and more inter-connection), some users may face the issue of the Short Circuit (SC) level exceeding the SC rating of the switchboard such as during momentary parallel of 2 incomers or 2 transformers. So should one change the switchboard just to cater for that few seconds when you momentarily parallel 2 or more sources?

Note: Please read the following with the assumption that the switchboard SC rating is below the SC level of the system during momentarily parallel of 2 or more sources.

B. Risk:

i. If no fault during the switching, no issue.

ii. If there is fault during switching:

iia. Single phase to ground fault (earth fault).

This is the most common fault. For LV system, the earth fault in a directly grounded system may be as large or larger than its 3 phase fault level. However, for most HV (In Malaysia from 3.3kV up to 33kV) the ground is earthed through a Neutral Earthing Resistor, limiting the earth fault current much below the SC rating of the connected switchboard. Under this condition, the switchboard should be able to sustain the fault before the related breakers trip.

iib. 3 phase fault/ 3 phase bolted fault

This fault normally results from a progression of a single phase to ground faults. If this happens, the SC rating of the switchboard will be exceeded. The switchboard may be damaged beyond repair.

C. Probability of momentary switching and 3 phase fault happening at the same time.

The probability for a simultaneous 3 phase fault during switching is quite low, as most faults are single phase to ground faults. Your exposure time to the said risk is also limited, which only happens during switching.

D. What should you do if you face this issue (Switchboard SC exceeded) during switching?

• Replace the Switchboard - Best option is of course to replace the whole switchboard, but looking at the major capital investment, switching frequency in a year is quite low and the exposure time to the risk is only for a few seconds (during make before transfer), it is much more economical to have an Auto Transfer Scheme to control the exposure time to the said risk. Furthermore, for a 3 phase fault to happen exactly during switching is quite low, unless you wrongly close the earthing switch onto a live cable/ bus. Most faults are 1 phase to ground which is limited by NER in the system, and normally the earth fault is much below the SC rating of your old switchboard.
• Other mitigations required are to ensure that your switchboard are maintained periodically according to local regulations and OEM recommendations to ensure the integrity of your switchboard, adding a SC limiting device (I-s limiter, SC limiting reactor), or introducing instantaneous protection (this may only limit the SC from propogating to Steady State SC, Ik. Ipeak SC level is not reduced). When the time comes for your plant to be rejuvenated after 20 or 30 years, that would be the best opportunity to upgrade your switchboard to meet the required SC rating, thus eliminating the risk completely.

Picture credit to: Nikhil Shahu

https://unsplash.com/@nikhilshahu999?

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