z's reading above max tabulated value. HELP!!

[ADS]

Quote:ADS From reading the above, i thought it would be vital that Z's readings are below the maximum tabulated values to achieve disconnection time of an mcb within the given parameters.

Hi cmacca81,

I am one of the sparks who totally believes in the above statement, so for that you are right, in my mind.

I'm not one of the 'high Zs-bang a RCD on it' brigade

Are you sure you've read the other section properly, though.

Supplemetary bonding is, as it says, 'bonding'. It's used as 'additional protection' against electric shock - similar to using a 30mA RCD.

Supplementary bonding re-enforces your main protective bonding and helps to maintain the equipotential zone.

It works by re-establishing the equipotential reference at a particular location (e.g. a bathroom), between all extraneous and exposed conductive parts, to minimise the touch voltages between them under earth fault.

It, therefore, has nothing to do with 'earthing' - e.g. cpc, Zs etc.

'Exposed conductive parts' will sometimes be earthed to a cpc, but that is to earth the exposed conductive part, not to lower the resistance of the cpc.

 

[SLIPSHOD & SLAPDASH]

Did you zero your meter ?

 

[cmacca81]

Thanks ADS, what you have said makes total sence to me now. Thats all i needed to know!!

 

[cmacca81]

zero the meter for no trip loop test?

 

[SLIPSHOD & SLAPDASH]

zero the meter for no trip loop test?

Definitely yes , especially with those Fluke leads

 

[ADS]

Did you zero your meter ?

Definitely yes , especially with those Fluke leads

Is this a requirement with a 'fluke', Slapdash? - I can't say I've ever used one.

I know with my Megger there is no requirement to zero the leads.

 

[SLIPSHOD & SLAPDASH]

A loop test is a resistance test, so you are adding the value of the leads into the equation if the meter is not zeroed. These little leads should not make a difference to the overall measurement but they do, especially with dodgy Fluke leads. Our Fluke plug top lead has about .60 ohm resistance.

Also try the plug a few times before dismantling, its sometimes just a dirty connection on a disused outlet

 

[SLIPSHOD & SLAPDASH]

Adding to Ads (no pun intented) post, you can add an additonal Cpc to a circuit to lower the Z, maybe thats where your confused.

 

[sparkytim]

Quote:ADS Firstly, RCD'S are not fail safe devices, they are electromechanical in operation and can malfunction, but they are a valid and effective BACKUP to the other methods. They must not be used as sole means of protection.

From reading the above, i thought it would be vital that Z's readings are below the maximum tabulated values to achieve disconnection time of an mcb within the given parameters.

Second, For example, suppose a 45a bs 3036 fuse protects a cooker circuit, the disconnection time for that circuit cannot be met, and so supplementary bonding has been installed between the cooker case and an adjacent central heating radiator.

The above is where i got the idea that, as it says, if dissconection time cannot be met supplementary bonding is used.

But that would make most TT installations NOT SAFE!!!

SEE BRB PAGE 50 REG 411.5

The IET has provided us with BS761 it does not really matter what other opinions are, work to IT and you will be compliant.

 

[SLIPSHOD & SLAPDASH]

Quote:ADS Firstly, RCD'S are not fail safe devices, they are electromechanical in operation and can malfunction, but they are a valid and effective BACKUP to the other methods. They must not be used as sole means of protection.

From reading the above, i thought it would be vital that Z's readings are below the maximum tabulated values to achieve disconnection time of an mcb within the given parameters.

Second, For example, suppose a 45a bs 3036 fuse protects a cooker circuit, the disconnection time for that circuit cannot be met, and so supplementary bonding has been installed between the cooker case and an adjacent central heating radiator.

The above is where i got the idea that, as it says, if dissconection time cannot be met supplementary bonding is used.

But that would make most TT installations NOT SAFE!!!

SEE BRB PAGE 50 REG 411.5

The IET has provided us with BS761 it does not really matter what other opinions are, work to IT and you will be compliant.

I have not read that book, but it is very wrong to bond to a heating pipe. I am sure there is a confusion with what i mentioned earlier that 'adding an additional Cpc to lower the Z reading is acceptable'. The additional cpc would run from the point of utilisation back to the same terminal as the circuit cpc and be labelled appropriately

 

[sparkytim]

There is a provision in BS 7671 to reduce the Zs by means of SUPPLEMENTARY equipotential bonding in addition to the CPC.

BRB Pg 47 Reg 411.3.2.6.

It was used more in 16th Edition days before more general RCD use when Earth Fault protection was provided by MCBs or Fuses ONLY and a low ZS was ESSENTIAL to enable the device to operate in the requred time.

With RCD protection Zs values ARE NOT SO CRITICAL, Hence a 200 Ohm ZS will still enable an RCD to operate within 0.4 Secs and comply, despite what some have been misled to believe.

 

[ADS]

There is a provision in BS 7671 to reduce the Zs by means of SUPPLEMENTARY equipotential bonding in addition to the CPC.BRB Pg 47 Reg 411.3.2.6.

Hi,

You have the right regulation - but the wrong reasoning.

The regulation is not saying that if you can't get a low enough Zs to achieve the disconnection times, then supplementary bond in order to reduce the Zs.

It is saying that if disconnection times can't be met, then 'additional protection' by means of 'supplementary bonding' is required.

It's similar to saying if disconnection times can't be met, use an RCD as 'additional protection'.

Neither of these are going to reduce your Zs figure - they just give additional protection

 

[ADS]

....For example, suppose a 45a bs 3036 fuse protects a cooker circuit, the disconnection time for that circuit cannot be met, and so supplementary bonding has been installed between the cooker case and an adjacent central heating radiator....

This example by cmacca81 highlights what I mean.

If the cooker circuit disconnection time cannot be met, then it is a requirement, as an addition to fault protection, to supplemetary bond the cooker to the radiator.

This is so that if the cooker casing were to go live under fault conditions, and we know it won't disconnect within the required time, then the touch voltages between the radiator and the cooker won't rise to a dangerous level as they are 'supplementary bonded' together.

It's not done to reduce your Zs or disconnection time - thats earthing

 

[sparkytim]

Hi,You have the right regulation - but the wrong reasoning.

The regulation is not saying that if you can't get a low enough Zs to achieve the disconnection times, then supplementary bond in order to reduce the Zs.

It is saying that if disconnection times can't be met, then 'additional protection' by means of 'supplementary bonding' is required.

It's similar to saying if disconnection times can't be met, use an RCD as 'additional protection'.

Neither of these are going to reduce your Zs figure - they just give additional protection

Yes I see that and another effect probably obtained by supplementary bonding would be to reduce the Zs due to parallel earth fault path and therefor reduce the time it takes for the protective device to operate.

Was it not a 16th Edition provision as an altenative if the Zs value could not be met to reduce the R1 +R2 by this means?

 

[ADS]

Yes I see that and another effect probably obtained by supplementary bonding would be to reduce the Zs due to parallel earth fault path and therefor reduce the time it takes for the protective device to operate.Was it not a 16th Edition provision as an altenative if the Zs value could not be met to reduce the R1 +R2 by this means?

You're right, it will have the effect of reducing the Zs - the more parallel paths, the better.

Maybe the Regs intended this double effect of supplementary bonding.

A bit like doing a conduit installation, measuring your R1 + R2 of the conductors, then attaching all your earth fly-leads to the conduit.

Your Zs will come right down compared to the calculated result.

Another reason why Zs should really be measured