Transformer Manhole and the $25,000 washer
This is a horror story from an electrical contractor about an event that took place around 1970. But the event reinforces the requirement to follow protocol to the letter when working on a large, say 50 Mva oil filled power transformer. This was a 30 MVA, 40 MVA, 50 MVA 138000 volts to 13800 volt oil filled power transformer. This is the story of a 10 cent washer making a 50 mva transformer useless.
It was not a MIDWEST project. And MIDWEST’s protocol is that everything is removed from the top of a transformer before the manhole covers are removed. Everyone has removed everything from their person and clothes and they must be wearing special coveralls and other secure clothing and all tools are secured.
In this case, all the bolts and washers for the manhole covers were left on top of the transformer and placed six feet away from the three manholes. The work on the 50 mva transformer was complete and the crew was lowering a manhole cover in place and “kind of dropped it” the last ½ inch. That caused one of the flat washers to bounce up, roll about 6 feet and plunk into one of the other open manholes on the top of the 138 Kv 50 Mva transformer. The chance of this happening was zero. Well almost zero. The transformer had to be hauled back to the manufacture for detanking to find the washer. A very sad day for that crew and a very expensive washer.
We don’t follow protocol just for what we know can happen. We also follow it for things we can’t image would happen.