Testing a light circuit

[mesh]

Hi,

Sorry for the basic question

I've started a C&g course and have a couple of questions;

I understand how to do a continuity test (R1+R2) on a lighting circuit for a 2 way switch , because I've been shown.
However I don't understand the reasoning and principles behind it - what am I really testing? Can you help explain? (in basic terms )

The other question I have is as above but for Insulation Resistance.

Thanks in advance,
mesh

 

[Adam_151]

You are determining the resistance of the phase conductor (R1) added to that of the protective conductor (R2), this is necessary becuase you are using EEBADS (earthed equipotential bonding and automatic disconnection of supply) to protect against indirect contact (contact with a part inadvertantly made live by a fault). In order for automatic disconnection of supply to occur in a prescribed time a big enough fault current needs to flow, there are graphs of this in BS7671, the current that flows is dependant on voltage over fault loop impedance (which is made up of two part;s the external loop and R1+R2), this earth fault loop impedance is termed Zs, and there are tables of values for it in BS7671, however these are maximum values at conductor opperating temperature, and when measurements are taken they need to be corrected for this, its normal to just compare the measured or calculated Zs value with 75% of the value)

The R1+R2 test is often used to prove polarity as well.

Insulation resistance testing is more intuitive, you are basically just testing the resistance of the insulation to make sure its erm.. insulating well enough :d

 

[rjb]

http://www.amazon.co.uk/Electrical-...1020402?ie=UTF8&s=books&qid=1193829306&sr=8-8

Buy yourself this book. It shadows 2330 levels 2 & 3.

All of your answers and more will be more.

Your question though. It is vital to have a low R1 & R2 as it makes up the value Zs (ze + r1 + r2) This is the earth loop fault. (Ze is impedance external to your installation)

In basics the lower the value the more current flows during an earth fault which means the fuse or CB trips quicker. (Regs stipulate that fuse/CB must blow/trip in 0.4s for socket outlet appliances and 5 secs for other circuits - lights would be included in this)

The regs give maximum values in ohms for a given fuse /circuit breaker in order to achieve these times.

Insulation resistance test is basically to test the circuit whilst dead to ensure there are no short circuits.ie ensure the insulation is good enough in your circuits ie your conductors are not touching value (min. 0.5M ohms) although if a reading is less then 2M Ohms then you should investigate your work!!


Incidentally, I answered a similar question yesterday in a thread called Multi-meter from Irish Sailor. There was some good stuff on that too. Have a look for it.

 

[mesh]

Thanks Gents, much appreciated.

I've read your replies several times, it is starting to make sense. I need to read them again as I have a couple of questions (hope you don't mind!). The questions are mainly on terminology.

I have also ordered a couple of books so hopefully (fingers crossed) that will help.

Cheers