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

[cmacca81]

Hi, i have just bought a fluke 1652 and thought i would have a quick go around the house. Whilst testing efli (z's) i found on the sockets it measured at 5.48 ohms. I checked in the onsite guide and discovered it should be a maximum of 1.24 ohms for a 30 amp mcb type 2 circuit breaker. I know that it's not going to meet the disconnection time. My question is, how can i get this down below the maximum tabulated value? would i have to connect a bonding conductor from a pipe of a radiator to the earth point of any socket outlet?

 

[NozSpark]

First off..

What's the earthing type?

What's the Ze?

Is it a RCD protected circuit?

 

[cmacca81]

supply is tns

will have to check ze (haven't opened up c/u yet, will check)

yes it's rcd protected (checked tripping time, 16.6m/s @ 5 x)

do you think it might have something to do with the supply earth? have you ever come across anything like this before?

 

[paul b b]

like noz says test Ze, also do a r1+r2 (Zs = r1+r2 +Ze) if youe ze is ok you may have a bad earth connection, may even be the insulation under the screw, so go round all your sockets tightining screws

 

[sparkytim]

Zs not relevent value for RCD protected circuit see BRB PG 50 table 41.5

120v -230 volt colume

30 m A RCD MAX Zs = 1666.66 (says 16667 but not quite right)

so your 5.48 is well in.

However this is an unusally high reading for sockets in a house unless its TT so check the R1+R2 as said and explore reason .

 

[cmacca81]

thank you so much for your replies, will check ze and r1 + r2 values. What if after finding that it is a bad earth but cannot locate where the problem lies (say a broken earth on one of the legs) What would you do, just out of interest? would you bond to the cpc of the socket?

 

[sparkytim]

Have you done a full Ring Test?

This will show up an open circuit CPC which could be your problem.

 

[Andy™]

whilst its within Zs of the RCD, because its TNS, it highlights a fault somewhere - most likely Ze is high. if so, you will need to contact DNO

 

[cmacca81]

hi, no haven't done a ring test yet. Only had a quick play with the fluke on rcd test and no trip loop test. After all your replies, i will start from scratch do all my cipper tests and hopefully identify the problem. just out of interest (as im still learning) i just wanted to know if electricians in general, if ze was satisfactory and there was an issue with the cpc somewhere, would you or can you bond to the cpc without having to rewire. would this be acceptable?

I appologise if my questions seem common sence, this is my only souce of information, that you can't really find in books.

Thanks again all

 

[sparkytim]

Not sure what you mean by "Bond to the CPC" from where to where?

 

[cmacca81]

From a cpc at the socket to a copper pipe of a central heating system

 

[ADS]

From a cpc at the socket to a copper pipe of a central heating system

Not in this lifetime

I'd do a bit more reading and revision if I were you - before you start running around with your new meter

 

[Andy™]

From a cpc at the socket to a copper pipe of a central heating system

what makes you think using a heating pipe as an earth for a socket is safe? either way, if the problem is external, it will make just about no difference

 

[Mr T]

[quote name='Andy

 

[Evans Electric]

Get that Ze checked first , its most likely the cause. Then go from there.

 

[NozSpark]

I could see the point of going from a C to a B, but not from a B to a C..

Off my head (for a 60898)

Max Zs (B32) 1.44 ohms (5x rated current to meet disconnection times)

Max Zs (C32) 0.72 ohms (10x rated current to meet disconnection times)

Max Zs (D32) 0.48 ohms (15x rated current to meet disconnection times)

 

[paul b b]

hi, no haven't done a ring test yet. Only had a quick play with the fluke on rcd test and no trip loop test. After all your replies, i will start from scratch do all my cipper tests and hopefully identify the problem. just out of interest (as im still learning) i just wanted to know if electricians in general, if ze was satisfactory and there was an issue with the cpc somewhere, would you or can you bond to the cpc without having to rewire. would this be acceptable? I appologise if my questions seem common sence, this is my only souce of information, that you can't really find in books.

Thanks again all

remember that lazy plumbers use plastic push fit pipes so bonding to a rad wont make a jot! what if some one rips out the rads? then it could leave the sockets with a high Zs.

but its most likely a high Ze or a loose terminal on the cpc at a socket

 

[Andy™]

Your thinking along the lines of supplementary bonding... the CPC is present but you are thinking of using a heating pipe to supplement the CPC in the circuit... 1 option to fall within the ZS is to change the circuit breaker i.e. from a B to a C or vice versa...

check the disconnection times for C breakers..

max Zs for a C is always less than a B of same rating... you could change C to B, or D to C. or any to a lower rating

but the reading here doesnt indicate bad design with a high Zs - it indicates a fault. which will most likely get worse

 

[sparkytim]

Your thinking along the lines of supplementary bonding... the CPC is present but you are thinking of using a heating pipe to supplement the CPC in the circuit... 1 option to fall within the ZS is to change the circuit breaker i.e. from a B to a C or vice versa...

check the disconnection times for C breakers..

No need re read Post 5

 

[cmacca81]

Quote:ADS

I'd do a bit more reading and revision if I were you - before you start running around with your new meter

It was information in a book that lead me to belive this in the first place, Maybe i have just missinterperated this information:

Taken from book Brian Scaddan (insepection, testing and certification)

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.

How the hell did i pass my 2391!!!