{Edited to remove misinformation [Linked Image] }

On the matter of RCD's, trip times and TT systems:

The legal requirement in Sweden for an RCD that is intended as a life saving device breaking the current within 200 ms. Modern technology has long surpassed this and the RCD's you can buy trip in about 40 ms. I think the Brits have upgraded their regs to require 40 ms.

Improved technology has opened a window for short time delay (or perhaps they should be called ultra short time delay) which waits 10 ms to see if the fault goes away. (Reduces nuisance tripping)

The RCD on a TT system is not intended to save your life directly, therefore it can be allowed to take longer to trip. As Paul indicated, if a fuse or MCB is used as protection in case of a earth fault it only needs to trip in 400 ms for 200-277V systems and 800 ms for 120V systems. I think the RCD has a 200ms requirement, though.

Note the logic: If the MCB is used as protection, the voltage to earth at the fault will only be half of the nominal voltage: The rest is lost in voltage drop in the line conductor. (Assuming same resistance in earth and line conductors) In case of a TT system, the voltage a the fault can be close to the full nominal voltage as the current is much lower. This means the more current will flow through anyone being in contact with the protective earth.

The manufactures have begun making RCD's time delay. (I think I've seen values from 50 to 300 ms)

This means that you can put two RCD's in series, where the "main" RCD only trips in case the other RCD fails. This in turn means that a house on a TT system no longer need to plummit into darkness, unless the fault is on a circuit which is protected only by the "main" RCD.

There are long time delay RCDs too, which means that you can have several "layers" of protection, just like with fuses or breakers.

The world is getting better each day!

Some good information on RCDs on TLC's website: http://www.tlc-direct.co.uk/Technical/Distribution/RCD.htm

[This message has been edited by C-H (edited 04-25-2003).]