Laminator blows fuses and trips the mains - but not when on my ispolation transformer

[dicky96]

Hi guys
So here is a 'weird device' it is an IKON IP-450 A2 Paper laminator

The customer says it was warming up then suddenly tripped out the 220V mains

He replaced the 5A fuse which had blown but it did the same thing straight away as soon as powered on, it trips out the power in his apartment. He tried another power lead and tried it at a friends address but the same occurs

So he brought it to me, having fitted a new fuse again.

I plugged it in and it worked fine - it was drawing about 3Amps and warmed up to maximum working temperature no problem in about 5 mins.

Having scratched my head as to why, it occurred to me the only thing I was doing different was that I had it connected to my 220V isolation transformer.

So I connected it to a normal power outlet, it blew the fuse instantly and tripped the mains out. The fuse is vapourised.

I then replaced the fuse, put it back on the 220V isolation transformer and it works fine

Thing is I can't see any shorts to explain this, either across the power or to ground. The heating element does read short circuit on my multimeter, but it obviously works OK.

The only thing I can see is one of the plastic terminal blocks that connects the power to the heating element has melted and looks a mess, but there is no short form this terminal to ground or to the screw next to the terminal block

So what's going on and why does it work on the isolation transformer?

[petehall347]

earth leakage ..isolation transformer won't have an earth .

[stj]

that or the isolation transformer is current-limiting

[Curious.George]

Originally Posted by stj

that or the isolation transformer is current-limiting

+1

3A @ 220V is 660VA -- probably a sizable load for an isolation transformer. If it was fused at 5A then it is likely that it draws close to that (at least at some point in its operation).

If there was a partial short (to earth or elsewhere), it would have to be significant (enough to make a difference between a ~3A operating current and a >>5A fuse trip current) for the fuse to "blow instantly" -- not likely something "moving" from thermal expansion, over time.

Put a light bulb in series with the load and measure current draw off the mains (w/o transformer).

[stj]

a lot of these things use a halogen lamp inside a metal roller, the cold inrush is huge!

[R_J]

I would disconnect the heat element and see if the short is still there, it could be that the heat element is shorting to ground, maybe only when it heats up.

[dicky96]

OK I can try with the heater disconnected

Just to clarify a few points raised...

My Isolation transformer is 220V:220V and rated at 2000VA continuous. It's good up to about 8 Amps (I know that from previous usage) It has a built in thermal trip and it did not trip out.

That connects to my Variac which is also rated 2000VA. On the Variac I had it set to 100% output voltage and the Laminator was connected to that. I do have a light bulb I can switch in series with the load but when i first turned on the bulb did not light so I bypassed it.

The Variac has linear voltage and current meters - that is how I know it was drawing 3A at 220V.

I had it on for a good 5-10mins until the led went green to signify the laminator is up to the set temperature. The owner who was there says this is normal behaviour. So I honestly don't think the isolation transformer is an anyway acting to decrease the load

To vapourise the fuse a lot of current must have flowed instantaneously when I connected it directly to the mains. My thoughts were the same as some here - a short to ground. But I can't see it on my DMM.

[redwire]

Look in the pic, the terminal block has melted and is shorting to the mounting screw.

[R_J]

With the laminator on the isolation transformer, when the short occurs you will be able to measure a/c voltage between a/c line and what would normaly be chassis ground. Without the short there should be next to 0 volts between a/c line and ground. And as mentioned above, it could be the shorting right at the white terminal strip

[dicky96]

Originally Posted by redwire

Look in the pic, the terminal block has melted and is shorting to the mounting screw.

Yes I thought exactly that as well. And as I posted on the OP I checked the terminal to the screw (or chassis) reads open circuit

[dicky96]

Originally Posted by R_J

With the laminator on the isolation transformer, when the short occurs you will be able to measure a/c voltage between a/c line and what would normaly be chassis ground. Without the short there should be next to 0 volts between a/c line and ground. And as mentioned above, it could be the shorting right at the white terminal strip

Yes, This. Very good suggestion, I will try that tomorrow. Thanks mate. I did check that white terminal already though, and it reads fine. Tomorrow I will remove it and fit something new just to eliminate it anyway.

[redwire]

Originally Posted by dicky96

Yes I thought exactly that as well. And as I posted on the OP I checked the terminal to the screw (or chassis) reads open circuit

Uh, give it a wiggle

[stj]

lol
and a good place for a ceramic terminal block and bootlace-ferules!

[dicky96]

OK it's fixed. It was that screw where it went through by the melted terminal block. It read open circuit from the terminal to the screw on my multimeter, even if I tried waggling the connector which actually would not move. But once I removed the screw and cut the melted part of the connector away I could see there was some bare metal inside and it had been arcing in there.

I didn't have any ceramic connector blocks so I soldered the wires and used heatshrink and then that white heatproof sleeving.

It works fine now

Rich

[budm]

BTW, Did you tighten all the screws on that terminal block, the one that melted was due to bad contact resistance so it got hot and melted the terminal black.