Maximum permitted Zs

[Ponty Massive]

Looking through the BRB and OSG regarding Max Zs (table 41.3), would these figures now only be applicable for circuits that do not require an RCD?

Table 41.5 gives max Zs for 30mA RCD as being 1667 ohms

 

[Des 56]

No the figure of 1667 is for TT systems
Tn systems are as listed
Table 41.5 is listed under the general heading of of 411.5 TT systems

 

[Ponty Massive]

Yes I agree this is for TT systems, but even on a TN system if the installation is covered by an RCD then wouldn't 1667 be the figure used?

 

[Des 56]

This is a copy of the opinion I gave, to the same question a little while back
Because TN systems have high fault currents

The overcurrent on a Tn is more important Zs wise than it is on a TT
The RCD and RCBO s electronics dont like high fault currents apparantly and are not as reliable as on a TT system
The Tn system with an RCD is not as reliable for the function of the RCd as a TT system

 

[pushrod]

the rcd is only in the event of an earth fault - in the event of a L-N short the rcd would not trip.
table 41.5 is only for earth faults. The low Zs ensures mcb operation in the event of a short.

 

[Des 56]

the rcd is only in the event of an earth fault - in the event of a L-N short the rcd would not trip.
table 41.5 is only for earth faults. The low Zs ensures mcb operation in the event of a short.

A better answer and more to the point

 

[robsparky1975]

To a certain extent yes,because the rcd is additional protection along as the zs is below 1667 ohms than its acceptable,although in practise your zs max on a tn system will be very low,the niceic inspector who looked at some new builds we done said after seing all circuits were 30ma protected said zs readings only needed to be noted under the clause of less than 1667 ohms,that was on a tncs,he saw ze,r1&r2 and was satisfied with just that,even though i took zs readings as well.with the on site guide quoting anymore than 200 ohms being unstable on a tt,should always be well within limits,just think of all those old tt installs with corroded rods and voelcbs out there offering next to no protection.

 

[Risteard]

To a certain extent yes,because the rcd is additional protection along as the zs is below 1667 ohms

Using an RCD (or supplementary bonding) because disconnection times cannot be met is not the same thing as additional protection.

 

[Des 56]

Rob
The jist of the post I made was that in TN systems,because the RCd doesn't work well with high earth fault currents,it cant be relied on for earth fault protection,so the Zs figures for TN systems should not be recorded or compared to the 1667 for TT systems,as pushrod pointed out the rcd wont see imbalance on a short circuit either

 

[robsparky1975]

Silly me,fault protection.

 

[trebor]

Looking through the BRB and OSG regarding Max Zs (table 41.3), would these figures now only be applicable for circuits that do not require an RCD?

Table 41.5 gives max Zs for 30mA RCD as being 1667 ohms

Table 41.3 is a mess and should be modified on the next reprint, the Zs figures are relevant to the OLPD only, the rcd characteristic is represented in Table 41.5 as the rcb part is nothing more or less than an RCD

The link below contains some very useful information from the manufacturers guild the BEAMA

www.beama.org.uk/download.cfm/docid/6BCA1329-8A17-4C89-B86A629F18D74BB8

 

[Ponty Massive]

Just changed a CU on a TT system and got a Ze of 1.09 ohms. Obviously well below the 21 ohms max value supplied by the REC. I just wanted to confirm this wouldn't be too low and my meter is working correctly?

 

[telectrix]

you must have pee'd some good beer residue on the earth rod to get that low a Ze

 

[malcolmsanford]

Somewhere in Australia Ponty there is someone hanging their coat on the end of you Rod mate

 

[Ponty Massive]

hahaha nice one. Not sure where the transformer is, but it's the lowest one I have seen for a TT