denkisyokku ga hituyouna toki, AED ga onseiannai wo suru.

Because 必要 is a na-adjective.

In Japanese, a na-adjective takes な when it comes directly before a noun. Here, the noun is とき meaning time.

So:

• 必要 = necessary
• 必要なとき = the time when it is necessary
• more naturally: when it is necessary

That is why it is 必要なとき, not 必要とき.

In the pattern X が 必要, が marks the thing that is needed.

So:

• 電気ショックが必要 = an electric shock is necessary / an electric shock is needed

This is a very common pattern in Japanese:

• お金が必要です = Money is necessary.
• 時間が必要です = Time is needed.

So here, 電気ショック is the thing that is necessary, which is why it takes が.

とき literally means time, but very often it works like when in English.

The part before とき describes what kind of time it is:

• 電気ショックが必要なとき
• literally: the time when an electric shock is necessary
• naturally: when an electric shock is necessary

This is a basic Japanese pattern:

• 忙しいとき = when busy
• 暇なとき = when free
• 日本へ行くとき = when going to Japan

So the first chunk of the sentence is a time/condition phrase.

Because this whole part is not a full sentence by itself. It is a phrase modifying とき.

Japanese often puts a whole descriptive clause before a noun. Here:

• 電気ショックが必要な describes とき

So even though English often expects a full clause like when an electric shock is necessary, Japanese builds it as:

• [electric shock is necessary] + time

That is completely normal in Japanese.

音声案内 means voice guidance, audio guidance, or spoken instructions.

With する, it becomes:

• 音声案内をする = to give voice guidance / to provide audio instructions

This is a common pattern in Japanese where a noun combines with する to make a verb-like expression.

For example:

• 案内する = to guide
• 説明する = to explain
• 音声案内をする = to provide voice guidance

So this part means that something gives spoken instructions.

That が marks the subject of 音声案内をする.

In other words, the missing word before が is the thing that performs the action of giving voice guidance.

So the structure is:

• 電気ショックが必要なとき、
• [something] が 音声案内をする。

That something is probably the device or machine being talked about, such as an AED, depending on the full context.

So this が is different from the earlier が:

• first が: marks what is necessary
• second が: marks who/what gives the voice guidance

Yes, that is a very good way to understand it.

The sentence structure is:

• 電気ショックが必要なとき = when an electric shock is necessary
• [subject] が = the subject
• 音声案内をする = gives voice guidance

So the overall pattern is:

• When an electric shock is necessary, [the device] gives voice guidance.

This is a very common Japanese sentence pattern:

• [time/condition], [main statement].

する is the plain form. In instructions, manuals, labels, and factual descriptions, Japanese often uses the plain form rather than the polite form.

So:

• 音声案内をする = plain style
• 音声案内をします = polite style

Neither is wrong, but the plain style is very common in written explanations and product instructions. It sounds neutral and matter-of-fact.

It is mainly there to make the sentence easier to read.

The comma separates the time/condition phrase from the main clause:

• 電気ショックが必要なとき、 = when an electric shock is necessary,
• [subject] が 音声案内をする。 = [the subject] gives voice guidance.

In Japanese, commas are often used more flexibly than in English. The sentence might still be understandable without it, but the comma helps show where the first part ends.

Sometimes Japanese gives you a choice between は and が, but here が is very natural in both places.

1. 電気ショックが必要

   • This is the standard pattern for saying something is needed.
   • X が 必要 is much more natural than X は 必要 in this kind of sentence.

2. [subject] が 音声案内をする

   • Here が marks the subject doing the action.
   • If you changed it to は, it would sound more like you are setting that subject up as the topic or contrasting it with something else.

So が fits the sentence well as written.