What is fail-safe design of switchgear control and relaying

[paul2gh]

Gentlemen,

What is the practice of fail-safe design of switchgear control and relaying? To be fail-safe, shall all dry-contacts be normally closed for interlock and intertripping? Shall all CBs be tripped when control power fails?

Is there any documentation regarding fail-safe?

Thanks,

Paul2gh

 

[slavag]

Hi.
Im not sure if possible found some documentation on the issue.
From practic, interlockings used with NC contacts ( possible with NO contacts too), relaying with NO contacts ONLY.

In some very critical systems used NV( No Voltage release for the CB with mechanical trip ), for example: MV motors, infeeds, but of course not all CB's.

Hope it help.
Best Regards.
Slava

 

[jasonf987]

Fail safe itself refers to a safe condition being achieved under failed conditions. This can vary depending on the application, just ask yourself, from a safety point of view what condition must occur should there be a failed condition? This will enable you to then decide on how the system should be connected.

 

[ScottyUK]

It's impossible to say what the 'safe' condition is without detailed knowledge of the wider process or loads. Taking your example of losing control voltage: from the switchgear perspective tripping the breakers would be fail safe (although what would you trip them with if you lost the T&C power...?) but if tripping the breakers resulted in (say) a huge release of toxic chemicals from a chemical plant served by the switchgear then the overall result is fail-to-danger. In that case it might be better to rely on backup protection elsewhere, or implement multiple fully-redundant tripping schemes.


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If we learn from our mistakes I'm getting a great education!

 

[oldfieldguy]

I agree with Scotty. We tend to be a little parochial when dealing with our pet power systems, but outside the substations there are things MUCH worse than breakers not tripping.

If you're worried about the control of your electrical system, then the idea (and costs) of redundant tripping supplies is a consideration but you will also need to deal with the process people as to what has to stay powered up for safety considerations. In my facility we have emergency power buses to care for critical valving and control functions. It gets to be a pretty involved discussion and is one that needs to take place early in the design process.

old field guy