RCBO 5In trip times.

[ste mac]

Just installing a new consumer unit for a customer. Split load board with 1 half RCD & the other half RCBO's. RCD & all but 1 RCBO has had good trip times, all sub 40ms at 1In & 5In, but the 6A RCBO for the kitchen lighting circuit is tripping at 40.8ms & 40.7ms at 5In. If I disconnect the outgoing live & retest the the trip times are 18.5 & 17.2. Anyone shed any light as to why this is happening & is it acceptable to test the RCBO without the outgoing ways connected?

 

[Guest55]

........ is it acceptable to test the RCBO without the outgoing ways connected?

Yes , and its the prefered method anyway.
Theres a good chance electronic transformers / ballasts are buggering up the readings at the first attempt at testing so youre good to go with recording the 18 / 17 ms trip times

 

[somersetsparks]

^^^^agreed, RCBO's should always be tested at their outgoing terminals with loads disconnected

 

[Jimmy Boy]

Biff/Somerset..Why ? Where is this written I can't say I have seen this anywhere although granted I don't own every GN book published, this is a serious question not a dig.
J

 

[Guest55]

Biff/Somerset..Why ? Where is this written I can't say I have seen this anywhere although granted I don't own every GN book published, this is a serious question not a dig.
J

No probs with asking jimmy lol.
The rcd manufacturers recommend testing of the devices in this way for a start.
Plus the test is just to acertain the trip time of the rcd , so theres no need for any other part of circuit to be connected.
And finally , electronic gizmo's can interfere with the readings , as shown by ste macs query.

 

[Guest55]

ps , i dont own any gn book lol.

 

[Crosswire]

Sounds like a bad earth on the kitchen lighting circuit to me.

Am not sure why you are disconnecting the lighting circuit to test the RCBO. I thought you had to measure disconnection times from the furthest point ( if not every point) on the circuit, so as to test the cabling of the entire circuit, and not simply the RCBO on it's own.

Or am I missing something?

 

[Guest55]

Sounds like a bad earth on the kitchen lighting circuit to me.

Am not sure why you are disconnecting the lighting circuit to test the RCBO. I thought you had to measure disconnection times from the furthest point ( if not every point) on the circuit, so as to test the cabling of the entire circuit, and not simply the RCBO on it's own.

Or am I missing something?

Er no.
Why test through loads of cable / switches / fittings which can lead to inaccuracies when you can just do it at the CU ?
Have a read of my post #5 for more info.

 

[Crosswire]

Surely if you test at the CU you will get a perfect reading every time?

If you test at the end of the circuit you will ensure a) the earth is continuous and b)the EFLI is low enough to operate the device in the required time.

Am not sure why you would want to ensure that the RCBO is fine, whilst the circuit and accessories that the device is meant to protect may harbour a fault that causes the (perfectly fine) RCBO not to operate in the required time?

I am not trying to be confrontational, I am simply confused as this is the first time I have encountered this method of recording trip times. I always record from the end of line.......

 

[Jimmy Boy]

Well that was my thoughts as well .
J

 

[Guest55]

Well you asked , i explained , do as you wish , makes no odds to me.

 

[Guest55]

Surely if you test at the CU you will get a perfect reading every time? Only if the rcd is functioning correctly , if its not then it wont pass the test no matter where you do the test.

If you test at the end of the circuit you will ensure a) the earth is continuous and the EFLI is low enough to operate the device in the required time. Eh ? this is a rcd test , what EFLI got to do with it ? that test is done seperately ?

Am not sure why you would want to ensure that the RCBO is fine, whilst the circuit and accessories that the device is meant to protect may harbour a fault that causes the (perfectly fine) RCBO not to operate in the required time? Then you have a problem with the wiring or accessories , not with the rcd , and you would need to investigate this seperately - your confusing what the test is for to be honest and the rcd is there to protect the user / occupier not the accessories.

I am not trying to be confrontational, I am simply confused as this is the first time I have encountered this method of recording trip times. I always record from the end of line.......

.......................

 

[Crosswire]

Don't know what to say to that........No offence was intended, purely questioning a technical point.

 

[Guest55]

No was offence taken , just trying to answer all your questions.

 

[Crosswire]

P.S. If the device is operating correctly with no circuit connected, but fails to operate in the required time with the circuit connected, the device has become pointless...Why have disconnection device that doesn't disconnect (in the real world) ?

I would check the earthing of this circuit again.