fault (see above, p 389); accordingly, if the non-performance is due to factors that cannot be attributed to the fault of the non-performing party he has, prima facie, a defence (PECL, Art 8:108). Only if such a party has agreed to guarantee a particular result will the plaintiff be entitled to damages irrespective of fault. In English law, the normative dimension is provided by the notion of promise itself; thus, all that a contractor has, prima facie, to show in order in order to obtain damages for breach is a breach of promise. Whether or not the defendant is guilty of fault is, prima facie, irrelevant (Raineri v Miles [1981] AC 1050, p 1086). However, as Vigers indicates, the English position is not quite so simple. Could it not be said that the undertaker was unable to claim in debt because he could not prove he was not at fault? To what extent is debt based on fault? 2 The position regarding fault in English contract law is not so simple because, although promise is a normative concept in itself, much will depend on what was actually promised. This problem has already been encountered under the heading of contractual liability (see p 389). In contracts where the object is a physical thing, the commercial supplier will usually be promising that the thing supplied will be reasonably fit and of satisfactory quality; if the goods are not fit, then there will be a breach of promise irrespective of the fault of the supplier (see Hyman v Nye, below, p 545, and Frost v Aylesbury Dairy, p 47). The supplier is liable because he is in breach of a promise now implied by statute (see the paradigm provision: Sale of Goods Act 1979, s 14, above, p 514). Where the object of the contract is a service rather than the supply of goods, a quite different promise is implied, as s 13 of the Supply of Goods and Services Act 1982 now makes clear (see p 391). A supplier of a service promises to carry out the service with reasonable care and skill. Accordingly, in order to show a breach of promise, the contractor suffering damage must show fault (that is, lack of reasonable care and skill). Is this the same requirement as for the tort of negligence? (Cf Attia, above.) 3 Where fault is relevant, upon whom is the burden of proving fault?
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Joseph Constantine SS Ltd v Imperial Smelting Co Ltd [1942] AC 154 House of Lords

This was an action for damages in contract brought by the hirers of a ship (the respondents) against its owners (the appellants). During the period of hire, the ship blew up for some unexplained reason. The Court of Appeal held that the owners could rely upon frustration only if they could prove that they were not at fault with regard to the explosion. The House of Lords allowed an appeal.