これは、火元が機体内部で、その周囲にも火が拡がったということですよね。周囲というのが筐体の外側を指すのか、発火源の周り（機体内も含めて）を指すのか、私には今ひとつはっきりしないのです。周囲ｘｘ以上と書くと、周囲よりさらにさらに火が回ったと読めるので、beyound the origin/initiating component and its surrounding area にしようかと思ったのですが、短くということでしたら、回答欄でも分かると思います。

参考になるかどうかわかりませんが、こんなのを見つけました。

http://www-pub.iaea.org/MTCD/publications/PDF/te_1134_prn.pd...
Use of operational experience in fire safety assessment of nuclear power plants

3.2.2.2. Fire severity factors
Severity factors can be used to overcome some of the difficulties that are experienced in
predicting the likelihood of damage using computer fire simulation or experimental data.
While experimental data can be used to predict the heat release rate from ignited combustibles
fire sources such as electrical cabinets and cable trays, tests have normally been conducted
with the aid of a substantial pilot fire. Likewise, while a computer simulation can be used to
determine whether a target will be damaged by a fire and how long it will take, the model
assumes the presence of an established exposure fire of a given size. In fact fires events do
self-extinguish, or are self-limiting in size. To therefore assume that all fires achieve worst
case predictions derived from experiments or assumed in the fire modelling is an unnecessary
conservatism. A severity factor approach is often used to determine a relationship between
conditional probability and fire size for each type of ignition source.
A concept for visualizing fire development was introduced by Berry and Minor in an
early approach to fire risk assessment [9], and may be helpful in classifying fire severity. The
four-stage classification scheme shown below is similar to that presented by Berry and Minor:
(1) Pre-incipient — ignition source present but does not succeed in igniting combustible,
(e.g. small explosion, overheated component, with no fire).
(2) Incipient — ignition of combustible occurs but fire self extinguishes with no damage
beyond initiating component, due to insufficient combustible/automatic de-energization
or lack of ventilation.
(3) Meaningful — self-sustained growing fire, potential damage beyond initiating
component.
(4) Severe — damage beyond initiating component requires fire brigade to extinguish;
significant cost to repair.

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Note added at 3 hrs (2008-01-18 05:23:52 GMT)
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うっつ、PDFファイルのテキストを貼り付けたので、改行が残ってしまいました。読みにくくてすみません。

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Note added at 11 hrs (2008-01-18 13:23:20 GMT)
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やっぱりそうですか。私が混乱した理由は、「筐体又は周囲に延焼」の「又は...に」なんです。これって、筐体に延焼、または（筐体も含めて）その周辺まで延焼、という意味なんですね。筐体には火が回らず、その外側に飛び火するってことないもん、と悩んだのでした。それなら、damage beyond the surrounding area としておいて、括弧内で　（spread to the case/chassis and/or its surrounding area） とすればいいかも。

私は筐体という言葉が好きなのですが、ITのマシン関係だと最近はこれを知らない人もいるようで、ケースとかボディと訳されていたりして、世代の差を感じます....