Help with answers not in regs book or in my brain please

[johno999]

Hi All,
Im hoping a bright sparks can help me with one or all of these 3 things that have been foxing me for a while
Any advice appreciated.
Thanks in advance

1) If im not wrong, a type B breaker must activate between 3 & 5 times its rated current within .1 second. There is time/current characteristics charts in regs book. But Im confused why you cant look up what would happen if for example a 32A breaker was to be running on overload at say 50A, as over time this would of course be dangerous.

2) In old days connecting low ohms meter for continuity test to a voltage would damage it. Assuming thats still the case, then does anyone make an r1/r2 test box that plugs in with voltage warning lights on it. As I figure it could be fairly easy to plug in to a live socket in error when doing continuity ring test. Unless of course you turn off the whole board. I could not seem to find such an obvious item for sale, or am I missing something?

3) If I need to do supplementary bonding in a bathroom for one reason or another, i.e no RCD protection. I think im right in saying that all extraneous conductive parts must be bonded to conductive parts. But is this only if they can be touched simultaneously. i.e I have a metal bath & the metal light fitting is at least 2.5 Metres away do I still need to join them with an earth cable?

 

[Andy78]

Hi Andy, Like I said Im studying & been out of game for a while, so hope what Im saying makes sense! But, Im guessing that if an RCD of 30mA as mainswitch is in a board then its giving you fault protection whether it was installed for that reason or not. So, if Im testing a circuit attached to a circuit breaker and the ZS is too high for that breaker, then why does it matter so much?

That would depend, as I said, under what circumstances the testing was being carried out and the earthing arrangement.

 

[telectrix]

back in the 16th, a 30mA RCD as main switch was generally fiited to give fault protection on a TTsystem where the Ze was too high to provide fault protection. however, final circuits should not rely on this as they should be designed to comply with ADS ( was EEBADS) using the OCPDs characteristics to provide ADS.

 

[johno999]

back in the 16th, a 30mA RCD as main switch was generally fiited to give fault protection on a TTsystem where the Ze was too high to provide fault protection. however, final circuits should not rely on this as they should be designed to comply with ADS ( was EEBADS) using the OCPDs characteristics to provide ADS.

Thanks.Unless anyone can shed more light on it, Looks like im gonna have to settle for, because the regs say so. I Just like things to make sense (-: If the RCD is going to trip anyway then it dont make sense to me that individual circuits are required to have zs in accordance with each breaker, unless of course its just a double protection necessity incase RCD failed

 

[telectrix]

end of the day. a RCD should not be relied on for fault protection. like on a car/van. the footbrake stops it. the handbrake is a back-up.

 

[Flanders]

Thanks.Unless anyone can shed more light on it, Looks like im gonna have to settle for, because the regs say so. I Just like things to make sense (-: If the RCD is going to trip anyway then it dont make sense to me that individual circuits are required to have zs in accordance with each breaker, unless of course its just a double protection necessity incase RCD failed

if you were carring out an eicr and you found a high zs but there was a 30ma rcd protecting the circuit that operated correctly then i would say identify this as a code 3 (improvent recommended ) but you would not normally design an installation this way mainly because rcds and rcbos can become unreliable expecially if the function tests are not caried out regularly. A comon thing i find is 32 amp type c rcbo used on long ring final circuits that often fail max zs values

 

[johno999]

good advice mr flanders, thanks guys

 

[PEG]

Also,unless i have mis-read,or missed something,your Zs readings are applicable to specific criteria.

Your RCD and MCB,are after all,tripping for different reasons :28: