قاطع الدائرة في صندوق المصاهر.

A natural way to divide it is:

• قاطع الدائرة = the circuit breaker
• في صندوق المصاهر = in the fuse box

So the whole sentence is a simple nominal sentence in Arabic:

• قاطع الدائرة = the topic / subject
• في صندوق المصاهر = the predicate, telling you where it is

Arabic often builds very short location sentences this way.

In Arabic, the present-tense verb to be is usually omitted.

So where English says:

• The circuit breaker is in the fuse box

Arabic simply says:

• قاطع الدائرة في صندوق المصاهر

This is completely normal in Modern Standard Arabic.
If you wanted a past or future meaning, then Arabic would use a verb, but for a present statement like this, no separate word for is is needed.

Literally, قاطع comes from a root connected with cutting or interrupting.

So:

• قاطع = cutter / breaker / interrupter
• الدائرة = the circuit

Together, قاطع الدائرة literally means something like the breaker of the circuit or the circuit interrupter, which is the normal idea behind circuit breaker.

So this is a good example of Arabic using a fairly transparent literal structure for a technical term.

Because قاطع الدائرة is an iḍāfa construction, often called a construct phrase.

In an iḍāfa:

• the first noun usually does not take الـ
• the second noun determines whether the whole phrase is definite

So:

• قاطع دائرة = a circuit breaker / breaker of a circuit
• قاطع الدائرة = the circuit breaker / breaker of the circuit

Even though قاطع has no الـ, the whole phrase becomes definite because الدائرة is definite.

This sentence actually contains two iḍāfa phrases:

1. قاطع الدائرة

   • قاطع = breaker
   • الدائرة = the circuit
     Together: the circuit breaker

2. صندوق المصاهر

   • صندوق = box
   • المصاهر = the fuses
     Together: the fuse box

An iḍāfa often works like English X of Y, but in natural translation it may become:

• breaker of the circuit → circuit breaker
• box of the fuses → fuse box

So Arabic uses noun + noun structure where English often uses an adjective-like noun before another noun.

Because Arabic is expressing the idea more literally as box of the fuses or box for fuses.

English often uses a singular noun before another noun:

• fuse box
• toothbrush
• car door

Arabic does not usually build such compounds in the same way. Instead, it often uses an iḍāfa with the actual noun meaning. So صندوق المصاهر naturally refers to a box associated with fuses, often multiple fuses.

So the plural is not strange here; it fits Arabic structure well.

Yes — دائرة originally has the basic sense of circle or something circular.

But in technical and modern usage, it also means:

• circuit
• electrical circuit
• sometimes even department or bureau, depending on context

This is common in many languages: a word with a basic physical meaning develops a technical meaning. In this sentence, the context clearly makes الدائرة mean electrical circuit, not circle.

في is the preposition meaning in or inside.

So:

• في صندوق المصاهر = in the fuse box

It introduces the location of the circuit breaker.

Also, because في is a preposition, the noun after it is grammatically in the genitive case. That affects the full formal pronunciation, even though short vowels are usually not written.

A careful pronunciation would be:

qāṭiʿ ad-dāʾirah fī ṣundūq al-maṣāhir

A few useful notes:

• قاطع = qāṭiʿ
• الدائرة is written with الـ, but because د is a sun letter, it is pronounced ad-dāʾirah, not al-dāʾirah
• صندوق = ṣundūq
• المصاهر keeps the l sound of الـ because م is a moon letter, so it is al-maṣāhir

With full case endings, it would be:

قاطعُ الدائرةِ في صندوقِ المصاهرِ

Here is why:

• قاطعُ = nominative, because it is the main subject/topic of the sentence
• الدائرةِ = genitive, because it is the second noun in an iḍāfa
• صندوقِ = genitive, because it comes after في
• المصاهرِ = genitive, because it is the second noun in another iḍāfa

In normal printed Arabic, these short vowels are usually omitted, but learners should know they are there in full grammatical analysis.

Yes, Arabic is somewhat flexible with word order, especially in simple sentences like this.

The given order:

• قاطع الدائرة في صندوق المصاهر

is a straightforward way to say:

• The circuit breaker is in the fuse box

If you moved the prepositional phrase earlier, that could shift emphasis, but the original sentence is the neutral, simple wording.

For a learner, the important point is that this is a standard present-tense nominal sentence: thing + location.