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Electricity supply protection...miniature version for single user?

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Flyback

Well-Known Member
Hello,
I was today powering a 150W offline LED light in the office. It’s a non isolated design, no transformer.
The PCB is on an earthed heatsink, but is insulated from it with a thin insulating sheet. There is no earth connection to the PCB itself.
Today it blew up, but strangely didn’t blow the 1.6A fuse in the AC line upstream of it. Some series resistors overheated and blew up (went open), so it just stopped working. The problem is, the blow up turned off the office electricity supply, and everyone’s computer went off.

Do you agree that installing an RCD wouldn’t have helped as the current couldn’t have flowed through earth (since earth isn’t taken to the PCB).
Do you know of any cheap protection units that I can put in my mains cable so it doesn’t trip the office electricity supply every time something blows up?
 

alec_t

Well-Known Member
Most Helpful Member
the current couldn’t have flowed through earth
Well, presumably it did, if an RCB was tripped. Could an inductive spike, when the LED light failed, have caused dielectric breakdown of the "thin insulating sheet"?
 

tomizett

Active Member
The teller will be what you had to switch back on to restore the supply - an RCD or MCB? The reason I don't like RCBOs is that you can't tell what tripped them.
Most (of the kinds that you'll encounter in domestic/office settings) MCBs will trip faster than a fuse, so it's possible that the MCB just went first - although in a race between a 1.6A fuse and a 32A B-curve MCB, I'd still expect the fuse to go first.

I'd recommend an isolated (and possibly variable [eg, "variac"]) AC supply if you're working with off-line gubbins. The one I use has a 6A MCB, and the extra line impedance of all the transformers etc protects the mains supply in the event of a short at the output.
 

AnalogKid

Well-Known Member
Most Helpful Member
Too many TLAs - three letter acronyms.

MCB - magnetic circuit breaker
RCD - ???

Magnetic breakers are available with trip characteristics from slower than slo-blo fuses to faster than fast fuses. Fast magnetic breakers are not cheap, but a 2 A or 3 A breaker in sereis with the LED light should protect the rest of the office.

A single 150 W LED light in an office?

ak
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Your description is incredibly vague - lights and power sockets should be (ARE!!!) on completely different circuits - a fault in a light shouldn't affect the power sockets feeding the computers (or any other mains sockets).

So please specify exactly what 'blew' - the options are presumably an MCB (but the lighting shouldn't trip a ring main MCB) or an RCD (again, an RCD on the ring main shouldn't trip due to a lighting fault). Possibly, from your vague description, you were powering the light from a mains socket?, for testing purposes? - which is a bad idea if you're using the same ring as the computers.

As for the fuse not blowing, a 1.6A fuse won't blow until long after a normal MCB - MCB's are far too sensitive, and trip if you give then a stern look!!.

But whatever happened, it shouldn't have - not in a correctly designed installation - you should run computer circuits separately, on a separate ring, with a separate MCB (or even better, an old wired fuse). Circuits protected by RCD's should be separate to the computer circuit, which 'could' (I personally would prefer not) have it's own RCD.
 

ChrisP58

Well-Known Member
Too many TLAs - three letter acronyms.

MCB - magnetic circuit breaker
RCD - ??? Residual Current Device. What is known in the USA as a GFI or Ground Fault Interupter

Magnetic breakers are available with trip characteristics from slower than slo-blo fuses to faster than fast fuses. Fast magnetic breakers are not cheap, but a 2 A or 3 A breaker in sereis with the LED light should protect the rest of the office.

A single 150 W LED light in an office?

ak
 

AnalogKid

Well-Known Member
Most Helpful Member
As for the fuse not blowing, a 1.6A fuse won't blow until long after a normal MCB - MCB's are far too sensitive, and trip if you give then a stern look!!.
I don't know what the I2T (i-squared-time) response curve is for a normal breaker in an office, but I've used MCBs with trip delays equal to or longer than thermal breakers in MIL systems that had high inrush current and steady-state current greater than what thermal breakers can handle. CarlingSwitch was my favorite for COTS, Airpax for hard MIL.

ak
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I don't know what the I2T (i-squared-time) response curve is for a normal breaker in an office, but I've used MCBs with trip delays equal to or longer than thermal breakers in MIL systems that had high inrush current and steady-state current greater than what thermal breakers can handle. CarlingSwitch was my favorite for COTS, Airpax for hard MIL.
There are a number of different types, I 'think' the standard one is type B? - when we moved the workshop at work a few years ago I ensured that the workshop was fed on it's feed from the main fusebox, with a wired fuse fitted (no way would I accept an MCB).

This feed goes to a fusebox in the workshop, with a number of different circuits, including an un-switched ring that feeds the computer, networking, and various infra-structure devices - plus a permanent power 'soak test' point. This circuit has it's own separate MCB, type B.

The rest of the power sockets are fed from a switched supply ring (using a cooker switch) and a type C MCB - even with type C it randomly trips a couple of times a year.

The lights are obviously on their own, are LED types, and fed from a type B MCB.

Basically there are light switches near the door, and the 'cooker' switch. When you enter you turn then both ON, and when you leave you turn them both OFF - thus making sure that everything (that you want OFF) is turned OFF. The computers etc. are unaffected, both by the switches, and by the tripping of any of the other MCB's.

I did later on add another un-switched double socket, for running anything else as well - mostly because I'd got a double socket going spare, and the type B MCB that I replaced with the type C one after the type B tripped every couple of days. It was trivial to fit the socket next to the fuse box, and run a wire from the spare MCB - a simple spur, and about a foot of wire.

Notice there's NO RCD fitted, they have no place in a service environment.
 

Flyback

Well-Known Member
Possibly, from your vague description, you were powering the light from a mains socket?, for testing purposes? - which is a bad idea if you're using the same ring as the computers.
Yes thats right, thats what i was doing.
 

audioguru

Well-Known Member
Most Helpful Member
You should not use a cheap Chinese non-isolated lamp in an office full of computers. What is its Mean-Time-Before-Failure? A few days? Was the cheapo lamp certified to be safe? I doubt it, or else the certification certificate was fake.

To promote saving electrical power my electrical utility gave away compact fluorescent light bulbs for free. But they were all recalled and replaced because many dripped burning plastic. The Chinese manufacturer stole the certification certificate from a competitor so those light bulbs were never certified to be safe.
 

tomizett

Active Member
Too many TLAs - three letter acronyms.

MCB - magnetic circuit breaker
RCD - ???
Apologies, yes! My excuse is that I was in a hurry...
I forget that people in different contries have different names for things.

MCB = minature circuit breaker, the kind that you normally find fitted in consumer units in homes, one per circuit. Believe they are a combination of magnetic and thermal, and are available in different speeds: "A" is the fastest, "B" is standard, while "C" and "D" are used for things like motor starting.

RCD = residual current device, which breaks the circuit based upon detecting leakages to ground from the difference between Line and Neutral currents.

RCBO = residual circuit breaker with overload - a combination of the above 2 devices in a single package.

Hope this clears things up.
 
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