Transformer Primary Protection Tripping on Start

My own less than carefully considered solution (and I'm talking less than 3KVA), when the manufacturer doesn't specify fuse size, is the D Curve MCB.

Sometimes that has required one size bigger wire to satisfy short circuit current requirements for D curve.

Can you explain a bit more what you mean by

"Sometimes that has required one size bigger wire to satisfy short circuit current requirements for D curve."

If you, in your jurisdiction have a requirement, that in case of short circuit, the breaker must trip in less than 0.4 seconds. The D curve breaker needs more short circuit current than C curve or B curve, in order for the magnetic (fast) trip to happen.

So sometimes, if you want to meet such a requirement, changing from B to C, or C to D, could also require having thicker wires.

So here is the 20 million dollar question

Say you cant get a drip curve to handle the inrush of your transformer.

What do you do?

For example, a transformer with 24 X multiple for inrush

Even a K trip curve can only handle roughly 12 X for 6 - 10 cycle

Other then a protective relay obviously because that is cost prohibitive.

Whats the solution

Yes, sure.

The breaker isn't there to <...protect the transformer...>. The breaker is there to PROTECT THE WIRING!

The wiring is therefore designed and installed according to the national electrical code applicable to the country of the installation.

How is that the case.

IF you connect 100 amps worth of load to a transformer 10 Amp trasnformer yes the breaker is going to protect the trasnformer

Yes the breaker is to protect the wiring in general

But breakers are used for more then just that

To protect equipment and for local isolation

https://en.wikipedia.org/wiki/BS_7671

If the breaker doesn't protect the wiring, then don't worry, because the wiring will protect the breaker...

When did i say that the breaker doesn't protect wiring.

I very clearly stated that it does but it ALSO protects the transformer.

The amount of effort you put into wise guy answers would be much better spent explaining the real information that you very clearly have from many years of industrial experience. However, instead, you choose the latter, why i will never know

If in doubt, consult a qualified Electrician.

See my previous reply

If a D-curve won't do it, one step up is to use a small MCCB.

More $, but with the correct trip unit the inrush can be tuned out so the breaker won't trip.

Use one of the manufacturer's circuit-breaker trip curve applications to plot out the trip curve and confirm that it should work before you buy.

This is an MCCB?

What did you think i was talking about?

Not trying to be a wise guy, im serious lol

What did you think i was referring to?

It may be a USA vs IEC thing.

These are MCBs:

Non-adjustable, fixed trip curves (C and D most commonly used here).

And this would be a MCCB:

I have yet to see a MCCB with a 'C-curve' or a 'D-curve' although they can be had with a fixed trip unit. The key difference is that with the MCCB, you can generally adjust the settings to get a trip curve that fits the application, where a MCB is non-adjustable.

You are correct.

I am an idiot, thank you for clarifying that.

It is an MCB.

Let me ask you a question. I was talking to Schneider electric yesterday (applications guy).

He says, like you, MCCB do not have a trip curve. But he also said that the only thing that was adjustable was the rating, not the time delay. SO!

With that being said, how does an MCCB help the situation if only the Iinst is adjustable?

He suggested that I upsize the rating.

So I think I know the solution to my immediate problem.

I have another question that came up from this.

Say you want to derate a transformer. Say you have a transformer with a 100 full load capability. You want to limit it to say 50-60A.

How would you go about doing that if you can't do it with a mcb or mccb because of in-rush issue? You can obviously put a 50A breaker in to limit it because it will not allow for startup.

Thanks for all the answers.

I think that the Schneider guy is a bit mistaken.

I have found that Schneider's technical catalogues are a bit hard to find and extract useful information from at times, but there is far more adjustment available than just Iinst.

Here is a link to a GE mccb catalogue. Record Plus Catalogue

Check out the section on trip units. There is a variety available from just the simple Iinst adjustment through to the likes of the SMR2 which is adjustable for

- An overload protection (Lt) of0.4 to 1 times the chosen trip unit rating.

- A delayed short circuit protection (ST) with a wide current and time setting range and optional I2t bands. - A selective instantaneous device (I) with a wide setting range.

Now I know that GE isn't everyone's favourite brand. It isn't necessarily mine either, just one that I am familiar with. The other mccb manufacturers all offer very similar feature sets, including Schneider.

For ratings less than 250A, in may be necessary to use a larger frame size and choke it down with a lower-value rating plug. I would expect that an St delay would help to buck your transformer inrush.

As I mentioned in my first post, there is software available for free from GE, Schneider ... that plots out the trip curves and lets you twiddle the trip unit settings and see what the effect is on the trip curve.

Thats awesome

Do you have a link to the software somewhere? I cant seem to find exactly the one i need.

I'm not sure what exactly you're looking for here. But, there are several ways to protect from both surge and sustained overloads. If you can't get both kinds of protection from one unit, you might have to consider putting in a separate unit with some kind of time-delay/slow-blow trip for sustained slight over-loads (to allow for starting surge-currents). Even a cheap set of fuses could do that (unless there is a different kind of problem encountered here). Otherwise, you might have to consider limiting the loads that restart automatically, and restart them manually one at a time (limiting the start surge).

If we follow NEC Table 450.3 [A or B] the primary side breaker rating has to be 6*Ixfmrat. That means 24*Ixfmrat=4*Ibreaker. A breaker type C will trip in not less than 1 sec. In my opinion, if the breaker will not trip in 0.5 sec it will withstand the inrush current. A cable of copper conductor XLPE or EPR insulated could withstand 25*Irated for 1 second.

So here is the 20 million dollar question

How do we protect the transformer

Generally speaking, we know that the MCB or MCCB is to protect the conductor, ok , great.

But! Say we have a transformer that has a full load rating of 30A

If we put at 40A breaking in or something over the full load, yes that will handle the inrush and yes the conductor will be fine.

But if we run over full load for extended periods of time and the breaker doesn't open we are going to exceed the damage curve of the transformer.

So how do we protect the transformer from over heating.

The forum cannot know how the <...we...> protects its <...transformer...>, as it cannot see the design documentation nor the installation that has been installed and commissioned.

One can only hope that its protection is adequate...

A recent Google search on that particular topic produced "about 1,230,000 results".

In my experience, the primary-side MCCB protects the cable to the transformer, and it provides short-circuit protection for the transformer. Depending on the application, it could be a magnetic-trip only CB.
Protection against overloads would be by secondary-side circuit breaker or breakers. Because these do not see the transformer inrush, they can be C-curve or D-curve MCBs.

It does depend on the application, this is not the only way to do it, but it is a method that I have seen used in automatic factory machinery.