Floodlights keep tripping MCB

[StormyK]

Give us the Ze from the EIC and the approx length of the floodlamp circuit ,of course a proper EFLI measurement would have been preferable but as thats out of the question...

Ze=0.31ohms. Circuit lengths:
CU to switch approx 60feet + switch to lights approx a further 60 feet = Total 120feet. CU to switch is in 1.5mm (new wiring) switch to lights is in 1mm (existing wiring). There is also at least 1 (probably 2) junction boxes in the circuit.

Thanks
Keith

 

[Spark123]

You can also carry out the R1+R2 measurement by connecting disconnecting the live from this circuit in the CU, connect it to the earth rail. Switch the light switch on to complete the circuit, go to the remote light fitting and measure the resistance between L and Earth. Add this value to the Ze obtained from the EIC. It is preferable to use a resistance meter which can deliver iirc a 200mA current source although a multimeter set to ohms range will give you a good idea of the resistance.

 

[RF Lighting]

If my maths is correct I work out approx. 1.46ohms at the light fitting.

The maximum allowed Zs for a 6A type C breaker is 3.20ohms

A type C should be OK.

 

[Adam_151]

I reckon thats an ELFI of about 1.8 ohms , well in for a C6 or B10, but I wouldn't use a C10 without a direct measurement, also definatly well in for a 5A BS1362

 

[Spark123]

Just looking at the time/current charts for BS1362 fuses from farnell, looks to me like it needs 19A for a 0.4s disconnection which at 240v = 12.6ohms. Applying the 80% rule gives 10.1ohms. If I read the end page of the document it is already worked out, although the 3A figure being lower than the 5A is a bit backwards.
http://www.farnell.com/datasheets/55950.pdf

 

[jondiy]

Why not replace the 6A B Type mcb in the consumer unit with a 10A B Type mcb , as this will cover the 1.5mm cable , then replace your light switch (to the floodlights) with a switched fused spur and put a 5A fuse in there - this should cover you for start up current due to its slower operating time , and will also protect your 1mm cable from the spur to the lights from overcurrent.

If anybody begs to differ - then fire away , I'm just looking for the easiest way out for the person asking the question, save him going to the trouble of obtaining RFLI , PSC readings etc.

 

[Steve]

I reckon thats an ELFI of about 1.8 ohms , well in for a C6 or B10, but I wouldn't use a C10 without a direct measurement, also definatly well in for a 5A BS1362

I was reading this just now, thinking "wouldn't a cartridge fuse holder handle these surges better?"

 

[jondiy]

Ditto.

Thats why I suggested using a 5A Switched Fused Spur in my previous post!

I did'nt suggest the HRC carrier as it was not specified what consumer unit the person had , other that Wylex. I'm assuming its one of the newer NH type (which use NB MCB's).

As far as I am aware I think the HRC carriers only fit the older NS type units but somebody will probably misquote me on that.

 

[kai]

Please note not all consumer units have compatible fuseholders available instead of breakers.