>50v Preventing RCD Trip Timing.

[davetheglitz]

Personally I'd be in smug mode right now! Fault found, all working, and been open and honest about what you found. Respect!

Can't see how a ring continuity would help identify a spur off a ring. Cannot have been an R1 + R2 issue. Must gave been an insulation problem in the first place - but pressures to get things done - failing light - sh8t it ain't working - panic etc. Never been there before but I'm sure it must be awful!!!!!

Nice!!

 

[avinalarf]

sounds as tho the RCD is not reacting quick enough to earth to prevent a touch of >50v appearing on the CPC.can you do a ramp test with a dilog?

2 further questions...

What's an "S type" RCD? and

What's a "Ramp" test?Blushing

 

[kme]

Nat.

S type is a time delayed RCD - used for discrimination purposes.

Ramp test is where is steding increasing earth leak is "fed" to the RCD, until the unit trips. The meter then provides the actual current it took to trip. Useful for finding unduly sensitive RCDs

KME

 

[paulh]

Personally I'd be in smug mode right now! Fault found, all working, and been open and honest about what you found. Respect!Can't see how a ring continuity would help identify a spur off a ring. Cannot have been an R1 + R2 issue. Must gave been an insulation problem in the first place - but pressures to get things done - failing light - sh8t it ain't working - panic etc. Never been there before but I'm sure it must be awful!!!!!

Nice!!

This is one of the main reasons you do all the cross connecting of ring final ccts in order to find spurs, reversed polarity's and continuity checks. A spur will always have an increased resistance compared to the other outlets in a ring. dave please read gn3 and all will become clear

 

[JimG]

Paul H - no offence taken. I didn't want to do an insulation test until I cleared the touch-volt issue, and this bleeding fused-spur was hidden from sight behind the back of the sink units. I came across it when I found the cable leading from the water softener thru to the kitchen.

I realise a spur has an increased impedence, but I'm unsure how you could read that from an R1/Rn/R2 reading from the CU?

JimG.

 

[kme]

This is one of the main reasons you do all the cross connecting of ring final ccts in order to find spurs, reversed polarity's and continuity checks. A spur will always have an increased resistance compared to the other outlets in a ring. dave please read gn3 and all will become clear

Silly comment mate. If you don`t know the FCU/SSO etc is there, you can`t test at it to determine if it is on the ring or not, now can you?

However, the insul. test would have found it.........

Paul H - no offence taken. I didn't want to do an insulation test until I cleared the touch-volt issue, and this bleeding fused-spur was hidden from sight behind the back of the sink units. I came across it when I found the cable leading from the water softener thru to the kitchen.I realise a spur has an increased impedence, but I'm unsure how you could read that from an R1/Rn/R2 reading from the CU?

JimG.

Taking the last point first.

You can`t. As noted above.

However. Order of tests (easy version)

1...DEAD tests.

2...LIVE tests.

Insulation readings are a dead test. The reason you do that test BEFORE live tests such as RCD are for this very reason. You find a problem before you energise a potentially faulty circuit.

The moral being - do the tests in the correct order. You would have found the problem quicker, and not been possibly creating a problem.

Suppose your RCD tester was of the older variety, and didn`t cut off the test at >50V to earth. You`re happily doing your test, out of sequence, and putting a harmful level of PD onto the exposed metalwork!

 

[JimG]

Oops - silly me, but thx for reminding me of the error of my ways.

I made the mother of errors and taken things for granted, bearing in mind this install looked only aro 3yrs old..... Lesson learned (again)

 

[paulh]

I guess it's time to be quiet then.......Guiness Drink

 

[steptoe]

I still think R1/Rn/R2 would/should have shown this up if conducted properly.

surely it was a poor R2 (E continuity) that was failing to allow the RCD to operate before the voltage rose above 50v...?

POOR ins res (L N) would have allowed the RCD to trip prematurely.... would it not..???

 

[davetheglitz]

Depends on the fault. If it's a L-E fault the trip would be quicker. However - if it was a N-E fault the then current which would normally go through the neutral is now shunted through the earth conductor. This means that the imbalance between Line and Neutral current is dependant on the load being taken. When testing I could well believe that this would cause the touch voltage to exceed 50v.

 

[kme]

Concur. Have had that happen a couple of times, when I`ve put a low imp. reading down to an undetected leaky load Duh to myself.

All goes to show that the sequence of tests, and following guidance does work. A;though, and I`m first to agree with steptoe here, real life can never follow the guide book exactly.

Paul. Don`t be quiet m8. If you hadn`t posted it, the point may never have arisen.

Consider yourself a leader in "public service"

 

[paulh]

Cheer's steps

 

[paulh]

If there was a neutral to earth fault wouldn't the rcd of tripped even quicker thus the touch volts would never of risen near the magic 50v?

Just can't keep quiet me

 

[davetheglitz]

Hi Paul.

Consider a TT system with an earth rod - Ze of 100 ohms. Now consider a 100ohm N - E fault on the consumer side. In the event of a fault the current that would go straight to earth is now split equally between the N and E. This means that at the RCD the imbalance between the line and neutral current is half what it should be - so for a 30mA fault only 15mA would be detected by the RCD.

Note this is under no load conditions as per the test. Assuming L-N impedance as around 1 ohm then under load the RCD would trip when a current of 3A is being taken.

As the N-E fault approaches zero, less and less current will flow to earth - so it is unlikely that the RCD will trip.

TT is the worst case - but a N-E short on a TNCS would halve the fault current under test - so likely that the touch voltage is exceeded - as the test is carried out under no load conditions. However as soon as >60mA is taken as a load the RCD would trip.

Weird isn't it!

 

[steptoe]

Hi Paul. Consider a TT system with an earth rod - Ze of 100 ohms. Now consider a 100ohm N - E fault on the consumer side. In the event of a fault the current that would go straight to earth is now split equally between the N and E. This means that at the RCD the imbalance between the line and neutral current is half what it should be - so for a 30mA fault only 15mA would be detected by the RCD.

Note this is under no load conditions as per the test. Assuming L-N impedance as around 1 ohm then under load the RCD would trip when a current of 3A is being taken.

As the N-E fault approaches zero, less and less current will flow to earth - so it is unlikely that the RCD will trip.

TT is the worst case - but a N-E short on a TNCS would halve the fault current under test - so likely that the touch voltage is exceeded - as the test is carried out under no load conditions. However as soon as >60mA is taken as a load the RCD would trip.

Weird isn't it!

yep, but we are on a ring final circuit so taking into account your calculations,,,,

plugging a 10a load into a socket has a N E split of 5a down each cable,

SURELY the RCD would trip ?:|

I like your theory, but sadly i think it is badly flawed,

also due to the fact that the touch rose above 50v would indicate to me more a continuity issue rather than ins res.

 

[davetheglitz]

Hi Albert - I thought that is what I said.

The example I gave for the TT system shows that the RCD would trip when the current taken was >3A.

Haven't quite got my head around the touch voltage bit yet - but I'm thinking!!!

 

[SPECIAL LOCATION]

This is one of the main reasons you do all the cross connecting of ring final ccts in order to find spurs, reversed polarity's and continuity checks. A spur will always have an increased resistance compared to the other outlets in a ring. dave please read gn3 and all will become clear

Silly comment mate. If you don`t know the FCU/SSO etc is there, you can`t test at it to determine if it is on the ring or not, now can you?However, the insul. test would have found it.........

I am inclined to agree with Paul H on this one...

The strict guidance for ring continuity includes three steps..

Step 1: end to end r1, rn, r2.

Step 2: L & N crossed read at each outlet.

Step 3: L & E crossed read at each outlet.

Pg 80,81,82 OSG

pg 36,37 GN 3

Even with an unknown faulty fused spur I think the readings would start to change around the ring as you tested each socket you find.. because the spur was switched ON with a water softener connected?

any items still connected would have some effect on the L & N crossed values? :|

 

[JimG]

Hey Paul H - you're a barrel of laughs aint yah?

 

[kme]

Given the possible load of such a device ( minimal, from previous experience), I wouldn`t be too sure of that.

Besides which, as you know yourself, real world scenario the readings have a variance. Even on a new ring, you`re highly unlikely to read the same result at each accessory. On an exploratory mission, a reading that was slightly out of sync might not ring the alarm bells in quite the same way???

Or am I talking "turkeys"??

KME

 

[davetheglitz]

Hi SL,

Can see what you're getting at - but I'm with Martyn on this.

Applying a few figures to this. Assuming L-L and N- N are 1 ohm. Cross connect and the resistance at any part of the ring should be around 0.5 ohm. If there was a load at the point of measurement of say 200 ohms (just over 1A) then this 200 ohms would be seen to be in parallel with 0.5 ohms - so the resistance would be 0.4987 ohms - a difference of around 1.3 milli ohm. This is too small a value to make any meaniful deductions of how the circuit is wired.