Old fluorescents and Newer switches.

[The Philpott]

Over the years, had had two incidents where modern light switches didn't last with old fluorescent tube (iron ballast) fittings.
One was a single 8' T12 tube (switch welded closed)
One was a pair of 6' T12 tubes (and the switch has now started arcing.)

Q.-Will careful selection regards quality of the light switch help..? Any brand recommendations? I never pause when i operate these switches, they are operated positively (and i am the only bod who operates them)

Q.-How would older (eg crabtree) light switches cope in this application? I have a couple of them but fear that despite their positive clunky action they will get arc damaged.

Thanks. Dave

[ValvoStef]

MK are pretty reliable switches. Don’t know about Crabtree.

[ValvoStef]

It is the inrush current due to the inductive ballast. There should be a capacitor in the luminar, parallel to mains to compensate the switch on surge.

[G6Tanuki]

I guess the issue here is the inductive-ness of legacy choke-ballasts.

"Old" light-switches were generally quick-make-break types with an over-centre toggle to give a positive snap-action. These are OK with iron-slug ballasts; modern switches tend to just use a cam-and-roller assembly which gives a slower contact-closure/separation and risks arcing.

What type are your proposed Crabtrees? They made both snap-action and slow-opening types. If they say "AC Only" somewhere on the base-plate then they are probably slow-opening ones.

Can you not arrange somehow for the switch to merely control the coil-current of a nice big two-pole contactor [interrupting both L and N] fitted in an earthed metal box somewhere in the attic? That way your switch only has to handle a small [though still inductive] load.

[emeritus]

The light switch controlling the industrial double 5' fitting in the garage failed a few years ago after some 30 years' service. It was a 1960's MK. As it was a two-way switch of which ony one pole was used, I turned the switch over and used the other pole.

[The Philpott]

..Will probably end up with an upgrade to a 'better' modern switch.
The older light switches i have are the round bakelite type kindly sent to me by a forum member, the round brown type with bakelite front and ceramic rear, the ones where the 'C' for crabtree is on the switch itself. (But obviously i don't want to ruin one unnecessarily.)
Thankyou for your answers.
Dave

[McMurdo]

Back in the 60's and 70's, the brochures for light switches often stated 'no need to derate for inductive loads'.

There is so much rubbish being sold these days that I've even seen a budget switch fail with the inrush from a 9W CCFL.

My workshop has 6 x 125W 8ft fluorescents and they use bog standard swiches, 3 on an MK pullcord and 3 on an unbranded RS rocker. No problems.

My kitchen here has an MK switch on a 65W fluorescent, been there 23-odd years. My dad's kitchen has a twin 85W fitting on an mk rocker switch, been there since the 70's.

I would treat yourself to a branded switch. MK, crabtree, Click, Volex, Schneider, Hager, you choose!

[trsomian]

Assuming that the fittings are the normal glow switch started type, then out of curiosity do the light fittings have power factor correction capacitors fitted; some domestic ones don't, because they are cheaper to make and domestic customers pay for watts, not VA like commercial customers do. I suspect yours may not.

The power factor correction capacitor tends to stop some of the voltage peak at switch off, albeit it tends to make the switch on worse by causing a brief current surge depending where on the AC cycle the contacts actually make. The instantaneous current surge in the inductor at switch on is limited slightly by the forward voltage of the discharge tube starter, and again depends on where in the mains cycle the contacts actually first touch.

[Lucien Nunes]

Use a good switch and you should get a few decades use out of it; there should be no need to use a vintage QMB switch. Rocker switch contacts are not all created equal. Some appear to be made of cheese, which is not good at withstanding aggressive loads. I fitted some 10AX rated (X = no derating for discharge applications) MEM grid modules controlling groups of 4x36W troffers and they started welding after about a year. As they failed I replaced them with MK and had no further issues.

[The Philpott]

MK it is then!

There is at least one large capacitor in the Twin 6' fitting as i recall.. (incidentally the starters are retrofitted solid state 'tube saver' items which give a timed start-up. Starts first time every time, as long as the tube is good.)

Dave

[broadgage]

Originally Posted by ValvoStef

It is the inrush current due to the inductive ballast. There should be a capacitor in the luminar, parallel to mains to compensate the switch on surge.

MOST older fluorescent lamp circuits use a choke in series with the lamp and a capacitor accross the mains in order to improve the power factor. Closing the mains switch results in a very substantial inrush current as this capacitor is connected to the mains.

A lot of six foot and eight foot circuits are different and use a series capacitor in series with the choke and the lamp. Opening the switch has to break a very inductive circuit.

A good qaulity light switch should last decades of normal use with any type of fluorescent light.
If problems persist, I would use a double pole 20 amp switch. These tend to be more robust due to the greater current rating. Use both poles of the switch wired in series.

A very low supply impedance increases inrush currents.
A dusty enviroment can destroy switches if dust enters the mechanism and impairs good contact.
In a dusty enviroment, If surface wiring is used, I consider it better practice to take the cable into the bottom of the mounting box, rather than the top. This reduces dust ingress.

[emeritus]

The impact of switch-on surge current was brought home to me some years ago when I put up a double 5' skip-find industrial fluorescent fitting in the loft to supplement the 4 x 100W GLS lamps already present. I originally simply wired the new fitting in parallel with the exsting GLS lamps with their single switch, but found that the inrush currents from the 4 x 100W tungsten bulbs plus that from the fluorescent, added to the steady current drawn by the circa 150W of the upstairs hall and bedroom GLS bulbs, was sometimes enough to blow the 5A fuse wire in the consumer unit. I replaced the single switch with a double for independent switching of the fluorescent and GLS lamps and have had no further problems.

[kellymarie]

Switch troubles with fluorescent lamps is nothing new where my dad worked they replaces one of 2 light bulbs with a 5 foot fluro the original 1940s lights witch gave an alarming crack at switch off it only lasted another week then had to be replaced by a modern switch[. I'm almost certain the fact that his workshop was damp didn't help

[robin0577]

Originally Posted by ValvoStef

MK are pretty reliable switches. Don’t know about Crabtree.

Actually, I'm inclined to treat MK with caution these days. Their stuff used to be amongst the best but the quality has taken a major turn for the worse since they became part of Honeywell.

[Lancs Lad]

This thread has reminded me of when my older brother moved into his first house in 1980.

The wiring was pretty ancient, with round brown bakelite dolly light switches, mounted on wooden back boards with a very soft up-and-down action - certainly not a definite fast make or break 'click' operation.

There was a fairly new fluorescent strip-light in the kitchen, that, upon being switched OFF, would quite regularly blow the downstairs lighting fuse in the very dated consumer unit (with rewireable fuses) which could easily be heard, because the fuse always blew with a heck of a spitting sort of bang. And the actual fuse/fusewire carrier was very scorched.

My brother was very tempted to just replace the 5 amp fusewire with some nice beefy 30 amp fusewire to 'remedy' the fault, but he never actually did such a silly thing. He put up with it for (too many!) years until the house was eventually fully rewired - in 1987!

It never happened when the switch was operated to turn the light ON, though

[duncanlowe]

Originally Posted by Lancs Lad

It never happened when the switch was operated to turn the light ON, though

We had a similar issue in our brand new house, 21 years ago. Kitchen, under cabinet flourescent fittings, two fairly small ones on the same switch. On switch off there would be a 'bang' and the MCB would trip. Never happened on switch on, always switch off. The T&E cable had been taken straight to the fittings via the flex grip. The flex grip had distorted the insulation on the T&E enough to reduce the breakdown voltage but not enough to cause total insulation failure. So the voltage spike on switchoff caused enough of a breakdown to trip the overcurrent.

[usradcoll1]

Originally Posted by McMurdo

Back in the 60's and 70's, the brochures for light switches often stated 'no need to derate for inductive loads'.

There is so much rubbish being sold these days that I've even seen a budget switch fail with the inrush from a 9W CCFL.

My workshop has 6 x 125W 8ft fluorescents and they use bog standard swiches, 3 on an MK pullcord and 3 on an unbranded RS rocker. No problems.

My kitchen here has an MK switch on a 65W fluorescent, been there 23-odd years. My dad's kitchen has a twin 85W fitting on an mk rocker switch, been there since the 70's.

I would treat yourself to a branded switch. MK, crabtree, Click, Volex, Schneider, Hager, you choose!

In the US, there is seldom any problem with the single pole switches used on our domestic 120 volt lighting circuits, but industrial and large commercial applications, it's a different story. Most of the lighting is switched at the breaker panel. The breaker controlling the lighting is a special unit designated "SWD", switching duty.
With a mainly inductive load, there's a false voltage on the neutral to earth and the breaker get warm and will trip, even though, the line isn't overloaded.
That's from my experience from over twenty years as an industrial electrician.
Dave, USradcoll1

[Brigham]

Originally Posted by The Philpott

..Will probably end up with an upgrade to a 'better' modern switch.
The older light switches i have are the round bakelite type kindly sent to me by a forum member, the round brown type with bakelite front and ceramic rear, the ones where the 'C' for crabtree is on the switch itself. (But obviously i don't want to ruin one unnecessarily.)
Thankyou for your answers.
Dave

The Crabtree 'Lincoln'. My all-time favourite bakelite switch.

Two varieties; all brown, or brown with white ceramic base.

Beware of the AC-only types. They look shallower, feel dreadful in operation, and are all-bakelite, no ceramic base at all.

They also have a short break.

The AC/DC types can handle anything.