Sunday Times 2
August 17 blackout: A question on interlock
I was motivated to write this article because 41 years ago, while working as a Shift-Charge-Engineer at the then oil-fired Kelanitissa Power Station (KPS), I was responsible for a similar incident which, fortunately, did not lead to a blackout.
However, it nearly led to the incineration of the Switchboard Operator. It was prevented in the nick of time by an in-built safety feature in the form of a moving separation shutter (Former CEB General Manager Eng. Nihal Wickramasuriya and former Assistant General Manager Eng. Ranjith Gunawardena may well recall this incident – we were colleagues at the KPS).
Last week, the Sunday Times’ page-eight article implied that certain alleged operations on the part of an Electrical Superintendent (ES) resulted in a sequence of events that led to the island-wide blackout.
It appears that the ES, in the following order, (1) switched off the circuit breaker (CB) powering the busbars where some maintenance activities were due, (2) ‘earthed’ the busbar area (so that the maintenance work could be effected safely), (3) carried out the maintenance work and, (4) finally, switched on the circuit breaker without disconnecting the earth connection from the busbars.
If this is true, the fourth activity would have been the proximate cause of the blackout. What in my view is remarkable is how, as alleged, the ES was able to switch on the CB without first disconnecting the earth.
The general reader may not understand why this should be remarkable, but there are good reasons for raising such a question which are based on the engineering concept of an interlock — an item of hardware (mechanical and/or electrical/electronic) which prevents one operation being carried out until another operation is done. For instance, if your house has an old fuse type single-phase main switch of MEM manufacture, even if you unscrew the cover, it cannot be removed without switching off the device. In other words, the cover and the switch are interlocked. The reason is, if the unscrewed cover could be removed without switching off the device, the latter would expose ‘live’ parts, and this could be dangerous.
Such safety precautions are present in most hazardous environments. Was it absent in the particular Circuit Breaker (CB) in the Kerawalapitiya Grid Substation (KGS) and the associated earthing system mechanism? The ES would have been authorised to switch off the CB, and if the CB and the busbar earthing system operations were interlocked, he/she would have been able to switch on the CB only after disconnecting the earth which he/she had earlier connected to the busbars.
In the good old days, this kind of interlocking was effected using a mechanical key. For a situation such as the one described here, there would have been only one key for both the CB and the busbar earthing system mechanism.
When the CB is ON the key is trapped in the CB. As the key is necessary to carry out the earthing operation, the CB has to be switched off to release the trapped key which can now be used to operate the earthing system mechanism. Once the latter is done, the key is trapped in the earthing system mechanism, and to switch on the CB it is necessary to release the key from the earthing system mechanism; and this can be done only by disconnecting the earth which was earlier connected to the busbars. Apparently, the latter did not happen at the KGS (It should be noted that today it is not necessary to use mechanical keys; the same process can be effected electronically). Was there no system that interlocked the CB operation and earthing system mechanism operation or, more disturbingly, as hinted in the Sunday Times Article, was any interlock that was in place “BYPASSED”, or “DEFEATED” by the operator?
Once again, for the general reader’s edification, the concept of defeating an interlock can be described if we look at our old friend, the MEM main switch, with the cover removed. The ON/OFF switch, which is now in the OFF position, cannot be switched on. But, as any electrician knows, such devices have a small lever that is visible when the cover is removed, and by pressing this lever with the finger of your left hand, you can switch on the device with your right hand: pressing the lever constitutes defeating the interlock which otherwise prevents switching on the device. This is an example of what is meant by defeating or bypassing an interlock, and is usually done for testing purposes and only under supervision. Such operations can render any system hazardous.
The Sunday Times Article states inter alia that, “It is strange and not acceptable at all if this kind of protection and interlock bypass has not been done under the supervision of a senior protection engineer of the CEB.” Does this comment imply that, there was an interlock and, it has been bypassed?
In my view, an CEB Electrical Superintendent would not attempt to bypass an interlock unless instructed to do so by an engineer, and in this instance it is unlikely that such an instruction would have been given as there was no need to bypass any interlock. The question therefore is, was there any interlock, between the said KGS circuit breaker operating system and the busbar earthing system mechanism, in the first place?
In conclusion, it should be mentioned that human beings, not being machines, can always make ‘mistakes’ either deliberately or accidentally. There are two broad approaches to mitigating or eliminating the possibility of mistakes. The more difficult one is to change attitudes (which could involve extensive education etc.) of relevant staff. But, from an engineering perspective, in certain instances the better approach involves using technology such as an interlock. Such an approach ensures that certain kinds of mistakes are impossible to make (There is an excellent description of similar issues in the Harvard Business School Case Study, Alcoa (A) which is analysed in the University of Colombo, Faculty of Graduate Studies, Masters in Manufacturing Management programme).