Time Delayed RCDs

[hightower]

If anyone has time to explain how time-delayed RCDs function, or point me to an article that explains it I'd be very grateful.

I understand the principles behind a standard RCD, just very curious how that same design can then incorporate a time-delay.

Many thanks,

 

[telectrix]

a bit of insight here:

http://www.farnell.com/datasheets/308191.pdf

 

[hightower]

Thanks Tel, think that just explains about the requirements for discrimination in terms of when to use a time-delay (unless I'm missing something). I'm just wanting to know what it actually is inside the device itself that gives the time-delay functionality? Is it achieved with electronics or some other wizardry I'm yet to encounter during my studies?

 

[telectrix]

not sure of the theory of how they actually work. could be black magic ( electronics) or something simply as a bi-metal taking a few millisecs longer to part contact.

 

[Simonslimline]

Maybe worth asking a manufacturer the physics and make of of such a device. There must be some element inside a selective type that delays the device sensing the opposing magnetic fields cancelling each other out and hence the imbalance causing the trip coil to operate. A bi metallic strip may be the answer all be it a very sensitive one maybe?

I like the question and it shows a keen willing to understand how things work and why, not just that they do and that is enough.

Also if my understanding of the RCD itself is not correct can someone please show me the error of my ways. Thanks.

 

[Richard Burns]

This is something that I also do not know, however I would expect a physical device may be too variable to meet the requirements and I can conceive that a small capacitor in parallel across the sensing coil would, briefly, as it charged, prevent the power reaching the release relay. In this way the size of the capacitor would determine the delay time, as the current through the sensing coil increased the delay time would drop.

Even non time delayed RCDs may have a small delay to prevent nuisance tripping from fast transients.
Older RCDs had very fast trip times at 150mA and longer trip times at 30mA, but modern ones seems to take longer to trip at 150mA but trip in the same time as at 30mA, presumably some other form of electronic control is in place.

 

[Simonslimline]

This is something that I also do not know, however I would expect a physical device may be too variable to meet the requirements and I can conceive that a small capacitor in parallel across the sensing coil would, briefly, as it charged, prevent the power reaching the release relay. In this way the size of the capacitor would determine the delay time, as the current through the sensing coil increased the delay time would drop.

Even non time delayed RCDs may have a small delay to prevent nuisance tripping from fast transients.
Older RCDs had very fast trip times at 150mA and longer trip times at 30mA, but modern ones seems to take longer to trip at 150mA but trip in the same time as at 30mA, presumably some other form of electronic control is in place.

A variable capacitor perhaps? Im interested in the make of this device now. So much so i may even speak to a manufacture for more info.

 

[Richard Burns]

This is the inside of a non time delayed RCD, as you can see the sensing circuit has a circuit board between the coil and the trip. (which actually looks like a rectifier, perhaps)
The test resistor is visible at the front. The terminals on the right are the supply connections.