RCBO not tripping on 5x test

[maccarooni]

Testing a radial circuit today and Zs .89ohm x1 rcd 218ms and 202ms and 5x greater than 40ms

TNCS earthing arrangement.Zs .34, Pfc .71. back at house. Circuit no more than 12/15m to outbuilding. circuits from sub board no longer than 15m.

This radial supplying an outbuilding with a swimming pool in it. 2.5mm 3c swa feeding a metal 6 way Board in the outbuilding, glanded straight to the box. :red card . Will be replacing the exported earth with a rod, and rcd-ing at the outbuilding, but my question is why would this be happening.

Gone through all the obvious.

IR on all cables ok.

have 3 rcbo's on new board and only this one playiny up.

Replaced rcbo for a new one and still same problem.

If I rcd test at metal cu trip times as follows on both rcbo's 27/17ms

Any help would be most appreciated.

Macca

 

[Andy™]

certain appliances etc are known to stop an RCD operating properly. did you test it in isolation?

 

[maccarooni]

Thanks for the reply Andy,

Sorry for the late reply, Late one yesterday.....

Anyhow,

Back at the house DB, the rcbo trips fine, 27/17ms.

At the sub-board in the outbuilding the rcbo trips the same.

When I go to the end of each radial circuit, I get the trip times of 218ms and 202ms and 5x greater than 40ms.

Now At this stage it doesn't really matter as I will be TT-ing the outbuilding and rcd-ing down there so hopefully that will sort itself out, But for future I was wondering what could cause this problem. Now if like you say an appliance or something on the circuit would affect the tripping of the rcbo, would that in effect make the rcbo fail (or not) under fault conditions.

Macca

 

[Andy™]

Thanks for the reply Andy,Sorry for the late reply, Late one yesterday.....

Anyhow,

Back at the house DB, the rcbo trips fine, 27/17ms.

At the sub-board in the outbuilding the rcbo trips the same.

When I go to the end of each radial circuit, I get the trip times of 218ms and 202ms and 5x greater than 40ms.

Now At this stage it doesn't really matter as I will be TT-ing the outbuilding and rcd-ing down there so hopefully that will sort itself out, But for future I was wondering what could cause this problem. Now if like you say an appliance or something on the circuit would affect the tripping of the rcbo, would that in effect make the rcbo fail (or not) under fault conditions.

Macca

yes, the problem can cause the RCD to fail to trip when needed. after all, RCD test is a real life situation with current leaking to earth. if it doesnt trip then, it probably wont trip when someone is the path to earth. even though it complies with 7671 if you get it to trip, i wouldnt be too happy leaving something i know may not work - really, its the same as throwing a bucket of salty water over a rod to get a better reading, but its high again once you leave

 

[maccarooni]

Thanks again Andy.

Yeah like I said I will be going back again to sort this out and will post an update then. Customer is away so cant get back til next week.

:run

 

[spinlondon]

What are the Zs at the point where you are attempting to test the RCDs?

I have heard that some testers are unable to carry out the test if the Zs are greater than 333.333ohms, as they are limited to a test voltage of 50V.

 

[maccarooni]

Spin

Highest Zs on one of the radials in the outbuilding was .98ohm. Zs at sub board was .89ohm.

Like I've said at the board rcd test ok 27/17ms but once past this point 200+ ms on X1

Macca

 

[spinlondon]

How are the batteries in the tester?

 

[maccarooni]

Spin.

New batteries,

Have 2 rcbo's and 2 rcd's on the new board, all others tested fine and have used 2 different ones on said circuit and both give exact readings....

Macca.

 

[spinlondon]

Sorry, all out of ideas.

 

[maccarooni]

Went back yesterday to finish the job. New rcd mainswitch down at outbuilding trip times 22/17ms and mcb back at house. Haven't found a reason for the last times on the rcbo...... headbang .

Job's done.

Macca