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Wiring fires

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alec_t
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Post Number: 634
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Posted on Tuesday, 10 May, 2011 - 02:48 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Did anyone catch Richard Hammond's 'Engineering Connections' on BBC2 on Sunday? We were invited to believe that lamp dimmers, of the type which chop the AC mains supply, pose a risk of overheating the supply wiring because they 'distort the AC current waveform'.
I find this hard/impossible to believe. Ok, the waveform *is* distorted and there are harmonics generated which require power factor correction in large installations (such as in the Burj al Arab hotel which featured in the programme, and which had capacitor banks for this as we were shown by Hammond), but surely the *average* current in a wire over a mains cycle is reduced by a dimmer, so the average power dissipated in the wiring resistance is also reduced? In other words, if the wiring is correctly rated for full lamp power without significant heating then there will be even less heating when the lamp is dimmed. The only dimmer risk I can see is that voltage spikes might cause wiring insulation breakdown; but this wasn't even mentioned in the programme.
Any thoughts?

Alec
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dave_g
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Post Number: 673
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Posted on Tuesday, 10 May, 2011 - 07:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Alec - I agree entirely. I have known dimmers themselves overheat, normally because they are overloaded. That might easily happen in a Hotel when, for example, someone fits larger "wattage" lamps that originally planned; mostly when there is more than one lamp on the circuit e.g. a chandelier or multi-lamp fitting.

Another issue is where the back box is too shallow, crimping wires against the dimmer PCB. Many of the cheaper dimmers are rubbish, quite frankly; I've seen the PCB hanging off a potentiometer with just tape for insulation. Add in a poor installation - perhaps Joe Public not changing the box from a 25mm one to a 35mm or 44mm, and trouble will ensue.

Apart from that, complete bunkum and I am rather disappointed in the otherwise excellent Richard Hammond.

The only other thing that occurs to me is that inductive loads - such as the older, wirewound lighting transformers - can be very large on lighting circuits, so the inrush current can be a big problem, and in the first days of low voltage lighting many electricians didn't take account of it. I've known that cause wiring damage.

NB - I'm going to switch on the PS3 and find it on IPlayer!

NBB - Just tried that, not available... :-(

(Message edited by dave_g on 10 May, 2011)

(Message edited by dave_g on 10 May, 2011)
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terry
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Posted on Tuesday, 10 May, 2011 - 10:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

http://ecmweb.com/mag/electric_fundamentals_harmonics/

Terry
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piers
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Posted on Tuesday, 10 May, 2011 - 11:00 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi all,

as seems to be common with others of this type of programme with an element of technical content, I suspect it has been over simplified to the point of bordering on incorrect, to avoid having the viewers eyes glaze over excessively.

Unfortunately in doing so fundamentals often get left out, leaving the explanation bascially correct but lacking and potentially misleading.

Striking the balance between correctness and conciseness cannot always be easy.

Piers

having just watched it on iPlayer via PC.

(Message edited by Piers on 10 May, 2011)
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stubble
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Post Number: 38
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Posted on Wednesday, 11 May, 2011 - 04:31 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

My dad is an electrician and is very reluctant to fit dimmers in any situation, he says itís not the wiring at fault, but the dissipated heat in the dimmer itís self, he says, how can you fit a large enough heat sink in the size of a domestic dimmer, fitted into a stud wall with flammable materials is asking for a fire to start, he will only fit them to block or brick walls.
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dselec
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Posted on Wednesday, 11 May, 2011 - 05:35 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello stubble
I have a 1000w dimmer installed in the wall and it does not heat at all or maybe 1w or 2w .
anyway dimmer or not NO!!! flammable material in the wall are allowed or maybe u could use 12 volts halogen or 3wattes leds
what is the energy he wants to apply for each dimmer

(Message edited by dselec on 11 May, 2011)
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stubble
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Posted on Wednesday, 11 May, 2011 - 06:54 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Itís something that he has never liked he has been an electrician for 40 years and seen lots of old and modern dimmers and still dose not like them. He has also seen poorly fitted units.
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dave_g
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Posted on Thursday, 12 May, 2011 - 07:53 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've now found it on IPlayer and watched the offending section (starts about 30 minutes in). What a load of rubbish! Anyone can set light to a bin soaked in Acetone with a thin wire, with or without a dimmer. I remain highly disappointed in the scare tactics employed by Richard Hammond, which will only confuse the consumer and waste everyones time.

With regard to dselecs comments, all dimmers will waste heat. It is similar to the power factor issue that was misconstrued in Richard Hammonds report. Heat is generated by current x voltage drop, which as we all know is complex with AC. The dissipated heat will depend on the size and nature of the load; but dimmers do get warm.

Stubbles' Dad is right, perhaps a little overcautious, but in a domestic situation you never know what the end user will fit after you've gone.
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joe
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Posted on Thursday, 12 May, 2011 - 08:12 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I did a very brief Google search just to see how many fires have been reported that are linked to dimmers and couldn't find any; but it was a brief search.

I did find this which at a quick glance seemed rather interesting and I'll have to have a proper read later: http://sound.westhost.com/lamps/dimmers.html

In Hamsters defence, he's probably not qualified one way or the other on this subject and was just following the script. The researches should probably be shot however.
Read my ramblings - www.techbites.com/joe-farr
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cjaysharp
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Posted on Thursday, 12 May, 2011 - 08:53 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I'd suspect that any fires caused by dimmers would have been caused by bad installation or plain out and out cheap rubbish devices.

From years ago when I was repairing nightclub/disco lighting controllers the usual failure was a shorted triac and/or blown fuses, never saw *burnt* devices unless someone had spilled a drink. Even the most overloaded and abused devices didn't fail with much more than a blown track, spectacular to see happen but not really dangerous if the case is fitted.
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dave_g
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Posted on Thursday, 12 May, 2011 - 06:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Joe - that link is truly excellent, thanks. I've read other parts of that website and the guy should be writing textbooks. Clear, well explained, very well presented, superb.

However, any excess current concerns would blow fuses before setting fire to wiring!
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alec_t
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Posted on Monday, 16 May, 2011 - 11:19 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've been faffing around with calculations and simulations. The only way I can get a dimmer (of the common leading-edge type, or of the less common trailing edge type) to draw more average current from the supply when the lamp is dimmed than when it is on full is if the lamp forms a predominantly capacitive load. For other loads (tungsten lamps, LEDs, inductive loads ) the average current is reduced (albeit not linearly) as the lamp is progressively dimmed.
So I think Hammond's scenario is, to say the least, most unlikely.

Alec
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violin
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Posted on Tuesday, 17 May, 2011 - 10:25 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Quote: Itís something that he has never liked he has been an electrician for 40 years and seen lots of old and modern dimmers and still dose not like them. He has also seen poorly fitted units. End Quote

Hi Stubble. There is absolutely nothing wrong with wall dimmer units as long as they are properly manufactured, properly tested and properly fittedÖ
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dave_g
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Posted on Tuesday, 17 May, 2011 - 07:49 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Violin - agreed. The problem is the DIY'er who does not understand what they are doing (many do, of course!) and create dangers.

The other problem is, for example, a 250w dimmer running 5 40w lamps in a chandelier. Then someone replaces them with 60w, a load above the dimmers capability.

Domestic wiring accessories is a very, very tough market for manufacturers and there is a lot of cheap imported rubbish. Add incompetent installation and you never know what will happen next.

Electricians are a cautious lot, and given the propensity to sue anyone and anything, tend to be very careful with what might happen after they've gone.
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echase
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Posted on Thursday, 19 May, 2011 - 10:09 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

According to datasheets for larger power dimmers/power controllers (1-5kW) they only dissipate one watt per one amp current. That is not a lot. E.g. http://docs-europe.electrocomponents.com/webdocs/0b69/0900766b80b69c7c.pdf But not sure if it is the same at all mark/space ratios and any power factor correction or suppressor components will add to dissipation.

Related to that if I powered a 3kW immersion heater from a power controller like this how much noticeable EMI would be radiated from the 10-20m of twin and earth that runs from fusebox to immersionand induced onto the rest of house mains? Would it meet domestic EMC standards? Whether the controller is at the fuse box or immersion end presumably makes no difference to EMI as itís the same distorted current flowing in both case.

(Message edited by echase on 19 May, 2011)

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