Thoughts on Temporarily Paralleling Two 1600A ACBs with a Bypass Castell Key?

The chief danger you have, is that the fault level will double, and it's unlikely the main switchgear (and design as a whole) has taken the double fault level into account ... if a fault were to occur during the time the transformers were in parallel, it could be a huge disaster, for example if the primary switchboard couldn't contain the fault level. If anyone were nearby the fault, and/or any equipment that was destroyed by the increased fault level, they might be seriously (or fatally) hurt.

Therefore, unless all of the possible outcomes are worked through, as mapj1 says, by someone with full details of the design and equipment, I would be extremely wary of this approach.

AMK said:

To avoid any supply interruption,

The usual approach to avoiding temporary supply interruption to key equipment, is to provide UPS ... if that has not been done, it seems like the design approach might have been for temporary shutdown in maintenance ... perhaps that is what is needed? 

As this maintenance is 'construction work' as defined, it's paramount the Client, and/or tenant, are fully aware of their duties in ensuring the maintenance is carried out safely - it's not simply down to the 'maintainer' to make this safe, and a requirement of 'power must stay on' might not be achievable if this hasn't been originally designed for.

The chief danger you have, is that the fault level will double, and it's unlikely the main switchgear (and design as a whole) has taken the double fault level into account ... if a fault were to occur during the time the transformers were in parallel, it could be a huge disaster, for example if the primary switchboard couldn't contain the fault level. If anyone were nearby the fault, and/or any equipment that was destroyed by the increased fault level, they might be seriously (or fatally) hurt.

Therefore, unless all of the possible outcomes are worked through, as mapj1 says, by someone with full details of the design and equipment, I would be extremely wary of this approach.

AMK said:

To avoid any supply interruption,

The usual approach to avoiding temporary supply interruption to key equipment, is to provide UPS ... if that has not been done, it seems like the design approach might have been for temporary shutdown in maintenance ... perhaps that is what is needed? 

As this maintenance is 'construction work' as defined, it's paramount the Client, and/or tenant, are fully aware of their duties in ensuring the maintenance is carried out safely - it's not simply down to the 'maintainer' to make this safe, and a requirement of 'power must stay on' might not be achievable if this hasn't been originally designed for.

As already siad above the proposal is risky.

You would aboslutly have to veify the 2 supplies have the same phase rotation and phase sequence. As alreay said there could be an issue with supply impedences and voltages leading to circulating currents.

As GK said 2 supplies may very well exceed the short circuit rating of the panel, bus bars and circuit protection.

I saw an installation where there were 2 transformers supplies suppyling a very large Form 4 panel with a bus coupler in the middle with no Castell interlocking of the 2 supplies and the bus coupler. I asked the AP if they had put the bus coupler in and he said no, I said if you are thinking about doing it let me get far away before you do, I did specify the refuge location but that would be giving the location away.

The interlocks are there for safety so overcoming a safty device is dangerous and possibly criminal. Find some other way would be my advice.

JP

Thank you for the responses. I agree with points about circulating currents etc.

The senior AP, from external contractors overseeing the HV maintenance, has stated that it’s common practice elsewhere to bypass the standard procedure (switching off one ACB, removing the Castell key, and closing the bus coupler). Instead, they routinely leave both ACBs energised, use a bypass Castell key to temporarily parallel the transformers for a few seconds, and then immediately switch off one ACB to avoid supply interruption. They claim this is standard in their operations and has been done without issues. I have checked phase rotation, and there’s no potential difference between phases of each busbar(Tx 1 - Tx 2)

Despite this, I’m still against proceeding with this approach.  Am I  being overly cautious ?, but given the potential consequences, I’d rather err on the side of safety. We’re responsible for issuing a permit to work per HTM 06-02 for the work. I’m not convinced this procedure meets that standard. Thanks again for the input!

You mention that you are referencing HTM 06-02 so I assume it is a hospital?

You are going to need a site specific Risk Assessment and Method Statement for this activity.  Comptencies of individuals and PPE will need addressing.

JP

Hi AMK,

Your external contractors may be indicating to yourself that its "Common Practice" but others may call that action a "Bad Habit"!!!!  Just because they say its common practice doesnt mean to say its actually correct and compliant. Reads as being far too risky to me, a few seconds doesnt matter, thats enough for something to go catastrophically wrong given the right set of the wrong circumstances.

Cheers GTB

You would absolutely have to verify the 2 supplies have the same phase rotation and phase sequence.

Indeed - cock that up and you are probably looking at waiting until something on the HV side lets go, and kills power to the site, and possibly a court appearance and a funeral to attend as well.

This reminds me that I have memories of standing next to the twisted metal of some large switch-gear that had been taken back to the old "Brush" plant  near Leicester for investigation, while a number of studious chaps peered at it hopefully rather in the manner of air-crash investigators.

The finer details of what had happened to get it like that had better remain under wraps, but basically due to a mix up about supplies. the fault current was at a level where the breaker could not actually break, and some other things were a bit slow on the uptake, and the arc had kept that current flowing for some seconds. The design lacked the (in my mind obligatory) 'death or glory' fuses at the incomer.  They are needed only for just this sort of thing.

It really did look very much as if a small bomb had gone off, nearly all the strain flaps had actuated - there are deliberate weak points in the enclosure that yield to allow the expanding hot gas and vapourised metal to escape without, hopefully, pushing the front panel in the direction of the operator, and instead directing the worst of the fireball out of the top and to a degree the sides.

Imagine the fairly heavy steel bulging alarmingly with inside-out crumple zones -don't under-estimate the power of the mis-placed electrons. 

In terms of the OP, unless it can be proven that the design is quite safe to be operated like that, whatever the contractors may do at other places, the sensible advice has to be 'don't '.

Mike.