Locating a 'short'...

[Mikey_A]

I want ot be able to calculate the distance of a 'short' on ring circuit. How do i change my IR reading of 0.00 megohm to the exact ohms?

 

[breezer]

dont use your "meggar" (unlless it has just an ohms reading)

 

[Mikey_A]

dont use your "meggar" (unlless it has just an ohms reading)

Ah, so if i switch to the low resistance setting......and use the figure recorded?

Cheers breezer

 

[breezer]

you can try.

but that is the theory.

BT have a really good DMM that really can do just that , its acurate to within 1 foot. Thats how they know where to dig the road up

 

[Mikey_A]

So how precise is the method i've mentioned?

 

[Adam_151]

BT have a really good DMM that really can do just that , its acurate to within 1 foot. Thats how they know where to dig the road up

I imagine its somewhat more than a low ohms meter tho, I'd guess TDR would feature

But yeah, if you have a dirrect short, your low reading ohm meter can give you a rough idea of how far along it is

 

[Taylortwocities]

The tables in the OSG will tell you resistance of the cable installed in milliohms per metre, you can then calculate a dead short as distance from one end, or the other.
Note: this only works for a DEAD short. If there's any resistance in the short it will skew your calcs.
But if its a ring you'll be able to find out where by testing at various points on the ring.

 

[Mikey_A]

OK then, just to clarify, let's say i had a 'short' between live and neutral. The resistance measured is 0.24ohms (measured at th CU).

Socket outlet, 2.5/1.5 t&e
Resistance in milliohm per metre = 19.51 (OSG)

0.24 ohms (expressed in milliohms is 240 milliohms)
240/19.51 = 12.30 metres approximately?

This sound right?

 

[nozspark]

No,

You are working out the L-E resistance and using it for a L-N fault.

The resistance of 2.5mm singles is 7.41m ohms/ meter which gives you 14.82m ohms/meter

So it's 240/14.82=16.19 meters

 

[Mikey_A]

No,

You are working out the L-E resistance and using it for a L-N fault.

The resistance of 2.5mm singles is 7.41m ohms/ meter which gives you 14.82m ohms/meter

So it's 240/14.82=16.19 meters

Of course, stupid mistake.

Thanks for pointing that out Nozspark.

 

[breezer]

i hate to poin this out, but as its two cables you are metering, would it not be half the given distance? (that the fault actualy is)

 

[ColJack]

he doubled the milliohms/meter to give the "feed" and "return" resistance..

if calculated with the 7.41 then yes, it's half the distance..

 

[breezer]

ok

 

[ColJack]

you can also use one of these..

 

[breezer]

perhaps thats what BT use (or similar)