Floodlights keep tripping MCB

[StormyK]

Hi, I'm having problems with 2 x 500w floodlights . They are wired in parallel and operated from a single (10A rated) fixed switch (ie not via PIRs). They are on a newly installed circuit on their own on an SP Type B 6kA (wylex NSB06) MCB. When I operate the floodlights via the switch, it trips the MCB. If I put the switch in the on position and reset the MCB they stay on. I can then use the normal switch to switch them on and off, but only for about 5 minutes when it then reverts to tripping the MCB.

The problem appears to be the startup load due to arcing across the switch. Any ideas as to how I can safely resolve the problem.

Do I need to change to a type C MCB which I believe withstand up to 1.4 times rated current for up to 0.5 seconds Is it safe to do this Are they interchangeable with and able to coexist with type B MCBs

Other alternatives may be
1) Change the bulbs to 2 x 300w (don't ideally want to do)
2) Increase the MCB size to 10A - however I am concerned about the safety aspect here (1mm2 cable)

Any ideas?

Cheers
Keith

 

[Steve]

I bought a GE bulb from B&Q a while ago, 300w it was, claimed light output of 500w. Im not dissapointed. Is there anything else on the same circuit as these 2 lights?

FYI, 1000w is approx. 4.34 Amps. Much else on this circuit and you will start tripping the MCB. And this is exactly why floodlights shouldnt be on normal lighting circuits.

You would have to ask an electrician to advise whether your circuit can accept a type C MCB. There are other factors besides cable size.

 

[RF Lighting]

I think a type C would be the best option, providing the Zs is low enough at the light fittings. (which it should be)

 

[Spark123]

Do you mean 6A MCB? 6KA will be the maximum value of current it can safely break. Halogens do cause a bit of a surge when starting, you could try a type C earth fault loop impedance depending, or revert to using a fuse (0.4s disconnection req).

 

[StormyK]

No, there is nothing else on this circuit at all. We have only recently (3 months ago) had the old Consumer Unit (most of the house has been re-wired) replaced with a split load (RCD) CU by a qualified electrician. It contains all Wylex Type B MCBs. However, I have only just completed the installation of the lights, hence he wasn't here when I found there was a problem.

 

[StormyK]

Do you mean 6A MCB? 6KA will be the maximum value of current it can safely break. Halogens do cause a bit of a surge when starting, you could try a type C earth fault loop impedance depending, or revert to using a fuse (0.4s disconnection req).

Yes I meant 6A.

 

[StormyK]

Do you mean 6A MCB? 6KA will be the maximum value of current it can safely break. Halogens do cause a bit of a surge when starting, you could try a type C earth fault loop impedance depending, or revert to using a fuse (0.4s disconnection req).

Pardon my ignorance, but can you explain further. I know the basics about electrics but am not an electrician.

"you could try a type C earth fault loop impedance..." - Do you mean carry out an impedence test (out of my skill range/no meters to test) or is this the description of a C type MCB.

 

[Steve]

methinks he meant "you could try a type C, earth fault loop impedance depending"

you would need ot know the ELFI to determine whether a type C would be able to break a fault in the correct disconnection time. Are these lights on a downstairs lighting circuit? If so, like i said, they shouldn't be.

 

[RF Lighting]

You need to test the earth fault loop impedance at the furthest point on the circuit.

If you fit a type 'C' MCB then the resistance to earth needs to be lower, so that in the event of a fault, more current will flow to earth, ensuring the MCB still operates quickly enough.

The difference is that with the type 'C' having a delay on a lower current fault then it will not operate as quickly as a type 'B' MCB.

I hope this makes sense to you. I have tried to simplify it as best I can for you.

 

[StormyK]

methinks he meant "you could try a type C, earth fault loop impedance depending"

you would need ot know the ELFI to determine whether a type C would be able to break a fault in the correct disconnection time. Are these lights on a downstairs lighting circuit? If so, like i said, they shouldn't be.

No. They are on a circuit totally on their own.

Am I right in saying that the simplest way to overcome this problem is to take the feed for these lights off the downstairs ring via a 5A fused spur unit? (ie disconnect from MCB). Am I still OK to incorporate the light switch (in the lower hall) after the fused spur switch (which will be in the cellar).

 

[Spark123]

oops, yeah that's what I meant Crafty. Funny stuff this punctuation eh???. Each circuit when designed must disconnect the supply within the specified time, MCBs only have one value for each type and rating which is the same for both 0.4s and 5s disconnection. For a type B 6A the max efli measured @20deg C is 240v/(5 x 6A) x 0.8 = 6.4ohms, for a type C it is 240v/(10 x 6) x 0.8 = 3.2ohms. What does it say for this lighting circuit on the EIC?

 

[RF Lighting]

What does it say for this lighting circuit on the EIC?

Good point.

StormyK, have a look on the certificate the electrician issued, and see what the value in the 'maximum measured Zs' coloumn is for the circuit with the flood lights on. This will tell you for certain wether the type 'C' MCB will be OK.

If you can't decipher the certificate, post a copy on here and we will tell you.

Swapping the MCB will be much easier, cheaper and generally the better soloution.

 

[StormyK]

What does it say for this lighting circuit on the EIC?

Good point.

StormyK, have a look on the certificate the electrician issued, and see what the value in the 'maximum measured Zs' coloumn is for the circuit with the flood lights on. This will tell you for certain wether the type 'C' MCB will be OK.

If you can't decipher the certificate, post a copy on here and we will tell you.

Swapping the MCB will be much easier, cheaper and generally the better soloution.

Unfortunately it doesn't have one for this circuit. As I stated earlier, this circuit wasn't completed when the CU was installed. Just connected to a junction box ready for final connection when lights were finally installed. Am I able to measure it simply myself with the right equipment? Will the spur unit method work OK?

 

[Adam_151]

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...

 

[RF Lighting]

Unfortunately it doesn't have one for this circuit. As I stated earlier, this circuit wasn't completed when the CU was installed. Just connected to a junction box ready for final connection when lights were finally installed.

Is there a value for the wire to the junction box?

Also is there a value for Ze? We will be able to get a good idea of the suitability of a type 'c' MCB from this.
The Ze value will be listed under 'supply characteristics' (on page two if it is a 'domestic electrical installation certificate' or page three if it is an 'electrical installation certificate')

Am I able to measure it simply myself with the right equipment?

With the right equipment, Yes.
You will need an earth loop impedance tester.

Will the spur unit method work OK?

Probably, but it puts quite a bit of load onto the ring circuit.
It is still alot better to keep the floods on their own circuit.

<edit> adam, as usual beat me to the Ze question