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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

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Old 30th Apr 2018, 9:26 pm   #101
philthespark
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Default Re: Vintage BS1362 plug fuses

A lot of complaints that I get these days as a professional electrician are about bulbs "blowing fuses", as someone else quite correctly stated, selecting the correct overcurrent protective device can be quite complex.
The reason why bulbs "blow" fuses in modern homes is that we actually use MCB's now, not fuses, however a lot of older people still call them fuses. In the older consumer units, usually circuits were protected by a rewireable fuse to BS 3036, modern units use MCB's to BS60898. Most normal MCB's react at around 1.5 times the rated current and react in a few milliseconds, a rewireable fuse can usually take aound 3 times the rated current before they blow, and react much slower,this is one of the reasons why it is important to make sure that the earth loop impedance value is below the limit for that particular MCB, and also why it is NEVER a good idea to replace for example a B type MCB with a C or D type without checking the loop value first, as it can vary quite markedly, not only between say a 6 amp and a 32 amp, but also between a B type and a D type.
At a given value of loop then in the event of a short to earth, or neutral then a certain amount of current will flow before the device trips, this can be as high as 2 or 3 thousand amps, for a few milliseconds if you have a low loop value, to as low as 5 or ten amps if you have a high loop value. Most modern wiring installations will be totally protected by an RCD, which to a certain degree makes the value of loop less critical, however should this fail, or one not be fitted, then there can be a real danger.
Let us imagine an installation not protected by an RCD, having a fairly high loop value, potentially we have the perfect storm. Imagine you accidentally drive a nail into a cable while securing a loose floorboard, and this nail only makes contact between the live and earth. now because you have a high loop value, you have only around say 20 amps fault current to earth, a single length of 2.5 twin and earth has a maximum current carrying capacity of around 27 amps, so because you have only 20 amps fault current, and your ring circuit is connected to a 32amp MCB,because the cable is a ring, then as you can see, the mcb will never trip but the cable has the potential to heat up and melt, causing a fire.
I went to a job last year and after carrying out my work, I did the necessary tests, and it was a good job I did, there was a fault on the cable feeding the property and the earth loop was 33 ohms, meaning it was unlikely that in the event of a fault any of the protective devices would operate.What was more worrying was that the fault, had, as it turned out later, existed for a number of years, yet despite other electricians having worked in the premises, non had noticed the fault, obviously because they hadn't bothered to test!
If you are carrying out any work on your home, or having it carried out by anyone else, please make sure it is tested, not only for the safety of you and your family, but also for the safety of your finances, if your house were to burn down due to an electrical fault, it's almost certain that the insurance company would want to see test certificates for any electrical work carried out, otherwise they may refuse to pay out.
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Old 30th Apr 2018, 9:46 pm   #102
McMurdo
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Default Re: Vintage BS1362 plug fuses

a light bulb in a table lamp is quite capable of blowing a 3A plug fuse. Lots of cheaper filament bulbs towards the end of popular incandescent usage are constructed in such a way that they arc over when they finally fail. Many quality bulbs incorporated a fusible link in the pinch to minimise this sort of thing but the last generation of tat often fail violently. In some cases I've seen them splatter the filament supports apart.
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Old 30th Apr 2018, 9:46 pm   #103
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Default Re: Vintage BS1362 plug fuses

After the sheltered housing occupied by my wheelchair-bound in-laws was rewired, their flat was plunged into darkness whenever a bulb blew. Their bulbs were from the local sainsburys and tesco supermarkets and had simple wire internal fuses. They had no further problems once I replaced all their bulbs with new ones from Woolworths. As I now have few woolworths bulbs left, I have have retained wire fuses to avoid the problem they had.

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Old 1st May 2018, 11:13 am   #104
philthespark
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Default Re: Vintage BS1362 plug fuses

Quote:
Originally Posted by McMurdo View Post
a light bulb in a table lamp is quite capable of blowing a 3A plug fuse. Lots of cheaper filament bulbs towards the end of popular incandescent usage are constructed in such a way that they arc over when they finally fail. Many quality bulbs incorporated a fusible link in the pinch to minimise this sort of thing but the last generation of tat often fail violently. In some cases I've seen them splatter the filament supports apart.
Years ago certain types of lamp had what was known as a "burn position", either cap up or cap down.You are quite correct in what you say about a fuse being built into some lamps, but an interesting thing with some of the cheaper ones was that when they failed, a piece of filament could fall onto the lead in wires creating a dead short resulting in the spectacular almost mini "explosions" that you mention.
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Old 1st May 2018, 11:52 am   #105
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Default Re: Vintage BS1362 plug fuses

I've certainly had a Poundland 100W bulb fail in quite an alarming manner in a wired fitting, though it didn't take out the consumer unit fuse (I still have fuses not breakers). It's made me very wary of bulbs made at the end of the incandescent era.
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