Can you get a shock with a 30mA RCD in place

[bernardgreen]

I once (when I was very much younger) held a live cable while standing on a rubber mat to demonstrate the lack of potential difference.

I recall the time an "electrician" (*) stated that any contact with a live wire would be fatal.

He was asked to explain how birds could perch on 11 Kv over head cables and survive.

(*) most "electricians" are little more than cable installers following a flawed book of rules. Flawed as it is the result of a poor attempt to create a fit all set of rules. The rules if followed can compromise the quality of some installations.

If electricians were taught and understood basic principles of electricity and how to use them then they would be far more efficient at diagnosing and locating faults. They could also determine when the rules will compromise the quality / safety / functions of the installation they are planning.

 

[sparkticus]

If electricians were taught and understood basic principles of electricity and how to use them then they would be far more efficient at diagnosing and locating faults. They could also determine when the rules will compromise the quality / safety / functions of the installation they are planning.

Could not agree more! Design and diagnostics often mean going back to basics (fundamentals) You can't go back to the basics if you have not studied them and don't know what they are.

 

[ban-all-sheds]

http://search.diynot.com/forum_sear...hor=ban-all-sheds&search=Search+Forum&stype=0

 

[JohnW2]

Having said all of that, you can now get a fast track medical degree in four years ....

Totally off topic (apologies) but I think that many people are probably trying to read too much into that. 40 years ago, the norm was 4.5 years for most medical students ...

For completeness, it's just occurred to me that the 'old'4.5 years and current fast-track 4 years are essentially the same. The 4.5 years actually consisted of 5 ordinary university terms (hence 50 weeks at most) of 'pre-clinical' studies, followed by 3 years of 48 week/year of clinical studies - which is almost exactly '4 years equivalent'.

Kind Regards,John.

 

[bernardgreen]

I have to stop my self from reacting when I watch an "electrician" trying to discover the route of cables to sockets in a building. Taking off front plates and disconnecting cables and doing continuity checks to other sockets and similar when in most cases it can be done without removing any front plates.

Just disconnecting the circuit at the CU and using some non standard (?) equipment.

By injecting a significant and constant DC current along the cable and measuring the voltage at each socket one can work out the order the sockets are connected along the cable. One can also work out which sockets are spurs and from which socket (or hidden junction box ) they are spurred from.

A similar principle enables the location of shorted conductors to be located.

 

[JohnW2]

By injecting a significant and constant DC current along the cable and measuring the voltage at each socket one can work out the order the sockets are connected along the cable. One can also work out which sockets are spurs and from which socket (or hidden junction box ) they are spurred from.
A similar principle enables the location of shorted conductors to be located.

Indeed, and BT engineers (and probably others) use tones, reflections and timings in much the same way - otherwise they'd be digging up roads and tearing houses apart even more than they do.

Kind Regards, John.

 

[Paul_C]

There are all sorts of methods which have been employed over the years for finding shorts and opens in telecommunication cables, from the traditional D.C. bridge methods such as the Varley arrangement for calculating the distance to a short or earth fault, to A.C. bridge methods employing tones and the capacitance to adjacent pairs to locate open circuits, to the modern time-domain reflectometer measurements which send a pulse down the line and then present a graphical display of the time taken to receive the reflected pulse from which the fault location can be determined.

The Post Office/BT Ohmmeter 18 is an example of a self-contained unit which can be set up easily for Wheatstone/Varley and A.C. bridge measurements, the various manufacturer's versions of the Tester 301 being a modern time-domain reflectometer instrument. A search on "Tester 301C" will bring up many references to the current versions.