fault finding

[Evans Electric]

Hi Justjess ,  hopefully to help you along .

One thing with faults is  ...stop & think & shut out everyone else who may be running about  , giving advice , causing confusion.   Lights were fine until this morning , so whats happened .

So the old elimination trick is always worth a go .    You already did that by finding it is the lighting circuit ...but its tripping the RCD ... NOT  the MCB  so a short is unlikely .

Theres leakage to earth on the lighting .

1)  Is there a faulty fluorescent fitting anywhere ?  Might be a ballast  or a cable routed past the ballast , melted insulation.

2)  Outside light full of water ?

3) Could try turning all the switches OFF  and try the RCD ..if it resets ..try switching ON one at a time until it trips again.

4)  As said above  could disconnect half the circuit  and try the RCD , continue to narrow the fault down . 

5)   Its always a good idea to ask whoever lives there if there was a bang somewhere  or a water leak somewhere or some DIY work carried out ,  owner has changed a light fitting ,  or has someone messed with the connections in the board ...moved the lighting neutral to the other bar .

Edit :  Forgot the one where a squirel / rat / mouse / wombat  has chewed the cables .

 

[Risteard]

Reg number?

For which bit? You've quoted both posts I made so I'm not clear which one you are enquiring about.

 

[Doc Hudson]

I am currently doing a level 3 electrical course. we are having to do a portfolio as we go along. the electrician i have been put wit isn't very helpful. i have done most of it but i am struggling with the fault finding. i just wondered whether anyone could help me with examples of electrical faults in domestic properties and the steps to diagnosing and rectifying these faults. thanks in advance.

The key ground rules of any fault finding are; Ask the owner/user all about the installation so you can establish as much as you can about any recent changes, alterations, or any symptoms that may point toward a possible cause of the fault. Obtain copies of any previous test results if they exist, but never assume anything is correct or working until you have tested and proved it yourself. When measuring voltages be 100% sure you know where the voltage you are reading is with reference to, i.e. floating or open circuit Earths or Neutrals, or reversed Lives and Neutrals have thrown many an electrician off down a wrong path. (we've had our fair share on previous questions over the years).  Always keep basic principles in mind if something starts getting confusing. Stop and think how you would verify it as a brand new just installed circuit, Dead tests - Live tests, sequence and expected results.  Never Ever do any test reading unless you know before hand what the expected correct reading should be. e.g. a cooker circuit Zs test reading of 1.30ohms  may be within the max permissible Earth Loop Impedance for a 32A 60898 MCB, (of 1.37ohms),  but if the installation Ze (TNS was 0.3) and the 6.0mm cable length was only 4meters (fuse box in garage backs onto kitchen)  Then Zs should be nearer 0.34.  higher readings than expected could indicate a lose termination or a damaged cable somewhere. Don't be a person who just presses the button and writes down the number without understanding what the number actually means or should be. Even without knowing precise cable routes, if you know how big a property is and the size of cables used you can have a reasonable idea about expected Max R1+R2 by using the tables in BS7671 (or On Site Guide)

Doc H.  

 

[Murdoch]

For which bit? You've quoted both posts I made so I'm not clear which one you are enquiring about.

That'll be me posting after a few glasses of vino...the 2nd bit...

 

[Phoenix]

5)   Its always a good idea to ask whoever lives there if there was a bang somewhere  or a water leak somewhere or some DIY work carried out ,  owner has changed a light fitting ,  or has someone messed with the connections in the board ...moved the lighting neutral to the other bar .
 

They never volenteer the information do they?!. Called to a takeaway once for lighting circuit tripping the breaker as soon as it is reset. L & N out the board and the there is a short showing L-E. I say I'm going to open some fittings to find it. Look for the middle of the circuit which seems to be the spice prep room, first fitting down is just connected with 1 T/E as a slave off another. Take another guess at which one its looped at, get it this time. And as luck would have it, choke is all toasted up and the 4xT/E run down the side are all melted into a mess..... Tell them I've found it and I'll just be a few mins while I sort it out (they were getting worried about not being able to open). The reply was something along the lines of, "Yeah, we thought we heard some bangs from that one" (never thought it might be usful to tell me though.... and I learnt to always ask comprehensive questions).

Got back to the yard and maintenance manager goes..... "They didn't try and pay in dead cats did they?"

 

[Risteard]

That'll be me posting after a few glasses of vino...the 2nd bit...

Sorry don't have Regs book on me at the minute but if you look below the table for the minimum measured values of insulation resistance it should be clear there.