A short story about when CPD won't work...

[Rockingit]

Just thought I'd share this quick sobering story that happened to me a couple of weekends ago....

Overhead 230v lighting festoon, LED golf ball ES, zig-zagging it's way merrily down a street in Central London used for market stalls underneath. Not my installation! High vehicle managed to catch it at one point [cautionary note about cable heights over roads..] and it snapped. Down came the source end, bare copper flailing about, sparking away as it crashed into an adjacent wet wall.... and stayed live, along with all the lamps up to that point.

Why? Because of two reasons....
1) being only a standard 2c L/N festoon cable there was no earth path for an RCD to be interested in and
2) because (and at this point I am surmising) there was so much load on the circuit because this line was so long (many hundreds of metres) that whilst all the drivers in the LED lamps coped with it fine, there was so much voltage drop on the existing load already that the thermal protection in the MCB just literally didn't see what by then was a few extra watts of energy. The CPD would have been a C16, so a max Zs of around 1.10Ω-ish and I'd hazard a guess that by this point in the chain it was twenty times that at least and probably had a voltage of less than 150v.

So it seems that when ADS disconnection time is all you've got left in the protective arsenal, for safe secs size really does matter.

 

[loz2754]

Interesting scenario. However I found myself wondering what Continuing Professional Development has to to do with it!
(The official abbreviation for overcurrent protective device being OCPD).

This comment is a product of my own OCD.

 

[brianmoooore]

I thought it was going to be a tale about being sold some fake weed.

 

[Rockingit]

Interesting scenario. However I found myself wondering what Continuing Professional Development has to to do with it!
(The official abbreviation for overcurrent protective device being OCPD).

This comment is a product of my own OCD.

Circuit Protective Device.

 

[Rockingit]

I thought it was going to be a tale about being sold some fake weed.

Now that would be CBD! This could go on for a while..........

 

[Rockingit]

Interesting scenario. However I found myself wondering what Continuing Professional Development has to to do with it!
(The official abbreviation for overcurrent protective device being OCPD).

This comment is a product of my own OCD.

OK, so every day's a school day etc.... It's not often that I get tripped up on terminology so I've just checked. It would seem that at some point back in time the generic term that I have always known as Circuit Protective Device (given the multitude of options that could be) has disappeared from the Definitions and looks like it's been replaced by Circuit Breaker. This doesn't really sit well with me because a circuit breaker can be anything from a sensing device through to a simple isolator, and OCPD doesn't always apply to every circuit. Apparently that's progress.....