R1+R2 ring refresher required please

[andyman 72]

hi there, i tested a ring circuit today and here are the values. r1 0.88 rn 0.87 r2 1.45 giving me R1+R2 value of 0.55

it was wired in 2.5 with 1.5 cpc , protected by a 32A rcbo type c, ze was 0.38 (tncs)

with this value it exceeds the value of 0.72 in table 41.3 but then according to table 41.5 the max earth fault loop for the bs en 61009-1 is 1667

would there be a problem with volt drop with these readings as i cannot determine the length of the circuit?Blushing

thanks guysBlushing

 

[ianmacd]

Why can't you determin the length? You have the size and the resistance of the loop, then you can work out the length (see OSG Appdx. 9).

Ian.

 

[ianmacd]

Have you measured Zs? Have you called the DNO about the Ze being over 0.35ohm?

 

[green-hornet]

Is this an actual installation or a question from an exam paper?

You can calculate the length of the cable by the resistance of the cable, tables are in the appendix of the big red book.

The maximum allowed Ze for a TNC-S is 0.35 so the reading of 0.38 is outside the permitted.

Voltage drop would be a problem as would the protective devise not working in the required time for this circuit.

 

[andyman 72]

sorry guys it is a tns, my mistake. brains not working well today. will check the big red book. thanks againApplaud Smiley

 

[SPECIAL LOCATION]

sorry guys it is a tns, my mistake. brains not working well today. will check the big red book. thanks againApplaud Smiley

Even if it was a TNC-S 0.38ohm - 0.35ohm = 0.03ohm

I wouldn't have panicked too much, could be just a error with meter tolerance and/or how good you poked your leads on.. do a re-check may come out as 0.34ohm!

BUT my question would be...

Why have you chosen a type 'C' RCBO? Why not use a type 'B'

Remember the figures in table 41.3 are MAX permitted at normal operating temperature not at testing temp...

rule of thumb take 0.8 of MAX values..

(see page 51 guidance note 3 or Appendix 14 big red)

so type 'C' is 0.72x0.8=0.57ohm max too low for your values

whereas a type 'B' would still give 1.44x0.8=1.15ohm max comfortable!

unless you have some specific loading requirements on the circuit,

power on sures, motors etc...

I would have gone Type 'B'? :| ?:|

 

[SPECIAL LOCATION]

hi there, i tested a ring circuit today and here are the values. r1 0.88 rn 0.87 r2 1.45 giving me R1+R2 value of 0.55it was wired in 2.5 with 1.5 cpc , protected by a 32A rcbo type c, ze was 0.38 (tncs)

with this value it exceeds the value of 0.72 in table 41.3 but then according to table 41.5 the max earth fault loop for the bs en 61009-1 is 1667

Table 41.5 is in the section realating to 'TT' systems,

see 411.5 higher up page 50

411.4 (back to page 47) relates to 'TN' systems

easy mistake, readin the wrong page weve all done it at some stage I'm sure...

I know I have in the past!!!BlushingBlushing

would there be a problem with volt drop with these readings as i cannot determine the length of the circuit?Blushing

thanks guysBlushing

2.5mm T&E ring.

1 conductor 2.5mm CSA @ 7.41mohm/m

1 conductor 1.5mm CSA @ 12.10mohm/m

{from page 166 OSG}

so your values give..

r1 @ 0.88/7.41 x 1000 = 118.7m

rn @ 0.87/7.41 x 1000 = 117.4m

r2 @ 1.45/12.10 x 1000 = 119.8m

I would say the ring is about 118m long approx!

For volt drop you need the actual max demand NOT the fuse rating..

whats going to be plugged in to it?

?:|

 

[ianmacd]

I thought most of the rcbo's out there were still type C and they are only just introducing B's?

 

[andyman 72]

top help there guys, thanks a lot

 

[SPECIAL LOCATION]

I thought most of the rcbo's out there were still type C and they are only just introducing B's?

NO type B's been out for a while now!

just some supplies are too Crap to supply em..

eg.. S-Fix

or even for example

"Electrifix" only list Type C Crabtree starbreaker RCBO's

but I fitted Type B crabtree starbreaker 10 months ago now!?

so why do not Mr & Mrs Fix bother to stock em???

tis a mystery? ?:| ? :| ?:| ? :| :|

 

[sparky999]

I thought most of the rcbo's out there were still type C and they are only just introducing B's?

Yeah i thought this to ian but i put in some steeple b types on friday so maybe there coming out slowly.

 

[batty]

Wylex are easily available at my main wholesaler. Mk are always B type and contactum are also B types.

Batty

 

[SPECIAL LOCATION]

Wylex are easily available at my main wholesaler. Mk are always B type and contactum are also B types.Batty

Glad someone else has actually seen Type b RCBO's:^O

Applaud Smiley

Tea & cake on Monday? :x

 

[batty]

Glad someone else has actually seen Type b RCBO's:^OApplaud Smiley

Tea & cake on Monday? :x

Deffo

 

[Andy™]

Glad someone else has actually seen Type b RCBO's:^OApplaud Smiley

Tea & cake on Monday? :x

i have some type B RCBO's (crabtree starbreakers)

 

[batty]

i have some type B RCBO's (crabtree starbreakers)

That won't get you tea and cakes you have not been a member long enough for the extras.

Batty

; \

 

[AK-UK]

My supplier used to have only type c, I insisted on type b and he brought it in, i can have now Hager, MK, contactum etc, all type B. Since 2004 I am using only RCBO configuration for my CUs and always could get the type B.

 

[Andy™]

That won't get you tea and cakes you have not been a member long enough for the extras.Batty

; \

i'll tell Admin if you dont share!

 

[christhesparky]

I thought most of the rcbo's out there were still type C and they are only just introducing B's?

We have been able to get hold of B Type wylex RCBO's for a couple of months now

 

[mcgaw81]

I have Hager type B and also a make called CPN again type B. They are good for the old type MK board, the small 6 way with the sloping cover. (if you understand that)