Eu desliguei o disjuntor.

Because the verb is desligar. In Portuguese, verbs ending in -gar usually add u before e in some forms to keep the hard g sound.

• desligar → desliguei
• compare with pagar → paguei
• compare with chegar → cheguei

If it were desligei, the g would sound different. So the u is there mainly to preserve pronunciation.

Desliguei is the first-person singular preterite of desligar.

That means:

• eu desliguei = I turned off / I switched off
• it refers to a completed action in the past

A quick comparison:

• eu desligo = I turn off / I am turning off
• eu desliguei = I turned off
• eu desligava = I used to turn off / I was turning off

You can absolutely say Desliguei o disjuntor. In Brazilian Portuguese, the subject pronoun is often omitted when the verb already makes the subject clear.

So both are natural:

• Eu desliguei o disjuntor.
• Desliguei o disjuntor.

Including eu can add emphasis, contrast, or clarity.

For example:

• Eu desliguei o disjuntor, não ele. = I turned off the breaker, not him.

Here, desligar means to turn off, to switch off, or to disconnect power.

It is a very common verb for:

• electronics: desligar a TV
• machines: desligar a máquina
• power-related things: desligar o disjuntor

Depending on context, desligar can also mean:

• to hang up a phone call
• to disconnect something
• less literally, to mentally switch off

In this sentence, it clearly refers to switching off the circuit breaker.

Disjuntor means circuit breaker.

It is the electrical safety switch in a panel that cuts the power when needed.

Examples:

• O disjuntor caiu. = The breaker tripped.
• Vou desligar o disjuntor. = I’m going to turn off the breaker.

In everyday Brazilian Portuguese, this is the normal word for breaker.

Because disjuntor is a masculine noun in Portuguese, so it takes the masculine definite article o.

• o disjuntor = the circuit breaker

This is something you usually just have to learn with the noun. There is no special meaning difference here; it is simply grammatical gender.

Desligar is the best choice here.

Portuguese uses different verbs for turn off depending on the thing:

• desligar: devices, machines, power, electronics, breakers
• apagar: lights, flames, things that go out

So:

• desligar o disjuntor = correct
• apagar a luz = turn off the light

You would not normally say apagar o disjuntor.

The normal word order is:

• Eu desliguei o disjuntor.

But Portuguese is flexible. You can also say:

• Desliguei o disjuntor.

That is very common and natural.

You might also hear other orders for emphasis, but the standard neutral order is: subject + verb + object

A rough English-friendly guide is:

• Eu ≈ eh-ooh or often more compressed in speech
• desliguei ≈ djez-lee-GAY
• o ≈ oo
• disjuntor ≈ djee-zhoon-TOR

A few pronunciation notes for Brazilian Portuguese:

• des- often sounds more like djez- in natural speech
• gui in desliguei sounds like gay, with the u not strongly pronounced
• j in disjuntor sounds like the s in measure
• the stress is on the last syllable of disjuntor

A more phonetic approximation of the whole sentence is:

• eh-ooh djez-lee-GAY oo djee-zhoon-TOR

Possibly, but disjuntor by itself just means breaker/circuit breaker. It does not automatically mean main breaker.

If someone specifically means the main one, they might say:

• o disjuntor geral = the main breaker

So:

• Eu desliguei o disjuntor. = I turned off the breaker.
• Eu desliguei o disjuntor geral. = I turned off the main breaker.

Yes, it is completely natural.

A Brazilian speaker would easily understand it as:

• I turned off the circuit breaker

Depending on the context, they might also say:

• Desliguei o disjuntor.
• Eu desliguei o disjuntor geral.
• Fui lá e desliguei o disjuntor. = I went there and turned off the breaker.

So the original sentence is short, correct, and idiomatic.