Step-down transformer tripping circuit breaker

[MindBender]

Hi,

I recently bought a 240V/120V step-down transformer to power my 120V JBC soldering tools from our 240V mains. It’s ha hefty 2000VA version, because both tools are 700Watt each, and 1500VA seemed a bit tight.

Now when I switch it on, even if it has no load connected to it, the circuit breaker for that circuit trips. It is an ABB, 16A B-profile with internal 30mA GFCI.

At first I thought it was tripping because of a ground fault, but disconnecting PE didn’t make a difference. However, connecting it to my isolating transformer did make a difference; The circuit breaker didn’t trip, but at switch-on the isolating transformer made a clean 50Hz hum that faded out in a few seconds, so it may be high inrush current, and a rather long one at that.

When I used my variac for the same purpose before I bought this step-down transformer, the circuit breaker also tripped every now and then. Both step-down transformer and variac are mono-transformers, transformers with just a single coil.

Does anybody know what’s going on here? And how I can fix this, if it is fixable?

Thanks in advance.

[MF-jockey]

The high inrush current is caused by the magnetization of the transformer.
https://youtu.be/kn9QOF5d-Sw
When I switch on a larger transformer, I use a series resistor (wire resistor 15 to 68 ohms, at least 10 W type) and a time relay which bridges the resistor after about a second.
The resistor should be mounted in such a way that it cannot cause a fire if the relay fails.
There is also a circuit variant with NTC and relay.
Then there is the luxury variant, the electronic transformer switching relay "TSRL" from FSM AG.
https://www.fsm.ag/de/trafosanfteinschalter/produkte/tsrl, sorry only in german.

[MindBender]

The high inrush current is caused by the magnetization of the transformer.
https://youtu.be/kn9QOF5d-Sw

Thanks for your reply!

The good news I take from this is that it is a very normal and well-understood phenomenon. Not well-understood by me though; The inrush seems to last longer than 1/4 for the wave period. I've watched the video, and I'm sure people with a better understanding of the integral formulae shown will understand why. But I'm not pursuing that understanding right now.

When I switch on a larger transformer, I use a series resistor (wire resistor 15 to 68 ohms, at least 10 W type) and a time relay which bridges the resistor after about a second.
The resistor should be mounted in such a way that it cannot cause a fire if the relay fails.
There is also a circuit variant with NTC and relay.
Then there is the luxury variant, the electronic transformer switching relay "TSRL" from FSM AG.
https://www.fsm.ag/de/trafosanfteinschalter/produkte/tsrl, sorry only in german.

Interesting! This TSRL comes in many variants, of which the simplest is exactly what you described; An NTC with a timer relay to bridge it:
http://www.trafo24.com/epages/79921744.sf/de_DE/?ObjectPath=/Shops/79921744/Products/TSR-0016
Unless you have all parts laying around (which I probably have) and you still know where to find them (which I probably do not), the price is pretty hard to beat for something pre-made, tested and working.

There's also a version that prevents the inrush current all together, instead of limiting it:
http://www.trafo24.com/epages/79921744.sf/de_DE/?ObjectPath=/Shops/79921744/Products/TSR-0009
I wonder if this is better for my application. It doesn't limit anything in case of over-current, the datasheet says, but NTCs have a limited life span. Decision paralysis...

[Psi]

Could put a incandescent lamp in series with in the primary then a relay across the lamp with the relay powered off the output of the transformer. Not the best solution, but its easy to build with parts lying around.

[woodchips]

Why not just change the MCB from the B rating to a D rating? D is for transformers, C for motors, B for lighting, A for resistive, just the amount of inrush current the load has.

[MindBender]

Could put a incensement lamp in series with it the primary then a relay across the lamp with the relay powered off the output of the transformer. Not the best solution, but its easy to build with parts lying around.

I recall when I was still in school, the loudspeakers for experiments in the physics room had an internal series light bulb for exactly that purpose. It's a nifty trick.

Why not just change the MCB from the B rating to a D rating? D is for transformers, C for motors, B for lighting, A for resistive, just the amount of inrush current the load has.

Well, those are expensive, but also; This MCB doesn't just power my office, but also other rooms in the house and that just doesn't seem right. And I wouldn't be able to use my iron in the other half of the house.

Anyway; I ended up ordering the TSRL from FSM AG. They seem to be very widely used.

[themadhippy]

Why not just change the MCB from the B rating to a D rating? D is for transformers, C for motors, B for lighting, A for resistive, just the amount of inrush current the load has.

Bit of a generalisation going on there.type A's are hardly ever seen in the wild,even on purely resistive loads,and if type B is for lighting what about low voltage lighting circuits that are transformer fed? And what about the earth loop impedance of the circuit,a type D  requires a much lower value  compared to a type B

[MF-jockey]

There's also a version that prevents the inrush current all together, instead of limiting it:
http://www.trafo24.com/epages/79921744.sf/de_DE/?ObjectPath=/Shops/79921744/Products/TSR-0009

By the way, this is also a TSRL from FSM, they have patents on it.

[MindBender]

By the way, this is also a TSRL from FSM, they have patents on it.

Ah; I was wondering about that during purchase, because I didn't want to buy an imitation. But it got here a couple of weeks ago, and it did indeed carry FSM and TSRL marks.

It is installed in the transformer housing and it works brilliantly! Thanks again for the tip.