OIL CIRCUIT BREAKERS
OIL CIRCUIT BREAKERS
(OP)
Searching for opinions with this one.
We are doing a study on an existing plant and have found the symmetrical breaking capacities of some indoor metalclad Hawker Siddeley 11kV OCBs to be marginal (18,4kA). The prospective breaking fault current has been considered at 5 and 10 cycles (50Hz) (including motor contribution) giving respective maximum overloads of 13% at 5 cycles and 5% at 10 cycles.
Upgrading of the breakers would be an extremely costly excercise and the client does not have the money available. The other option is of course fault limiting reactors but this is also costly.
It would be interesting to hear some opinion on whether the potential 13%/5% breaking overload is acceptable, bearing in mind that the breakers, although well maintained are pretty old (20 years). Does anyone have any knowledge of the old manufacturing standards and what factors of safety were used when rating the OCBs?
Thanks and best regards
We are doing a study on an existing plant and have found the symmetrical breaking capacities of some indoor metalclad Hawker Siddeley 11kV OCBs to be marginal (18,4kA). The prospective breaking fault current has been considered at 5 and 10 cycles (50Hz) (including motor contribution) giving respective maximum overloads of 13% at 5 cycles and 5% at 10 cycles.
Upgrading of the breakers would be an extremely costly excercise and the client does not have the money available. The other option is of course fault limiting reactors but this is also costly.
It would be interesting to hear some opinion on whether the potential 13%/5% breaking overload is acceptable, bearing in mind that the breakers, although well maintained are pretty old (20 years). Does anyone have any knowledge of the old manufacturing standards and what factors of safety were used when rating the OCBs?
Thanks and best regards






RE: OIL CIRCUIT BREAKERS
Once the risk has been identified, I don't think you can ignore it.
Just ask yourself if you would be able to explain your decision in court, or worse yet, on "60 Minutes".
RE: OIL CIRCUIT BREAKERS
RE: OIL CIRCUIT BREAKERS
Besides installation of current limiting reactors (or current limiters), it may be possible to reduce the fault level by operating with the buses split in such a way as to minimise fault currents in each section - I suggest that you explore that possibilty fully, as it could be a minimum or no cost option.
RE: OIL CIRCUIT BREAKERS
Those devices are available in the market for few years with several thousands applications. They operate under the principle of continuos monitoring both the momentary value of the current and the rate of rise in less than 3 ms according with the manufacturer performance data.
Could be installed at the main breaker or bus tie that limits fault current and is capable of handling high continuous and fault currents up to 36 kV. There is available rated voltage of 12 kV with 75 kV BIL and various continuos current rating from 1200 to 4500 A.
The down side is if there is any trip, the cartridges need to be returned to the manufacturer. Therefore, in the total cost of this installation the spare parts needs to be considered.
RE: OIL CIRCUIT BREAKERS
RE: OIL CIRCUIT BREAKERS
RE: OIL CIRCUIT BREAKERS
RE: OIL CIRCUIT BREAKERS
My biggest concern is obviously the safety aspect.
Due to operational constraints and loadings it is unfortunately not possible to reconfigure the network to bring down the 11kV fault level.
I don't see how current limiting fuses can work. If they are installed on the main incomers, any major downstream fault will take them into pre-arcing and result in total loss of plant. No co-ordination can exist with the downstream relay protection.
Once again, many thanks.
RE: OIL CIRCUIT BREAKERS
Another option (higher cost again) would be to install current limiters in parallel with reactors - normal operation is with the reactors shorted out & no voltage drop. After a fault the reactors remain in circuit to feed the buses, with the consequent reactor voltage drop.
RE: OIL CIRCUIT BREAKERS
If this solution did not work as planned, the design engineer would be holding all the cards, and all the cards would be bad.
I understand that the current-limiter makes sense from an engineering perspective, but I just wanted to point out that whoever recommends this solution is buying a huge amount of liability if something doesn't work right.
As an interim solution, you may want to recommend that the owner immediately restrict access around the under-rated breakers.
RE: OIL CIRCUIT BREAKERS