Last night’s session began with one PC escorting a prisoner (1). The PC (an artificer) wanted to have the prisoner in hand manacles, and further to have cast arcane lock on said manacles, both of which I allowed.
As described in the PHB, manacles have
Breaking them requires a successful DC 20 Strength check. Each set of manacles comes with one key. Without the key, a creature proficient with Thieves’ Tools can pick the manacles’ lock with a successful DC 15 Dexterity check. Manacles have 15 Hit Points.
Arcane Lock says:
While affected by this spell, the object is more difficult to break or force open; the DC to break it or pick any locks on it increases by 10.
The spell is easy to apply to a strength check attempt to break them: +10 to the DC is now DC30.
Likewise picking them is a now a DC25 Dexterity check.
However, the manacles explicitly have both a vulnerability to a Strength check and a listed hp as an object. Presumably one would use the strength check for an attempt which might shatter them at one go but which, if failed, would do minimal damage. The hp, on the other hand, would be relevant for several attacks which sought to accumulate enough damage to break them. It is worth noting that the listed hp for the manacles are well above the standard for an object of their size (see DMG 246, 247), which makes sense as their primary function is to not be broken.
Spells do what they say they do; my interpretation of “While affected by this spell, the object is more difficult to break or force open;” is that while the spell specifies the increase in DC for a strength check to break something, it would also increase the difficulty of breaking it by applying damage – but leaves the adjudication of that to the DM.
Others disagree, and contend that because the spell only mentions strength checks that is the only protection it applies. Answers to this question should assume that the spell does in fact protect against damage; frame challenges are useful only inasmuch as they advance arguments beyond what is already available at the linked questions (and @Darth Pseudonym, you are welcome to frame challenge – so long as you develop your answer more than what was already cited).
Assuming that the manacles already have an AC of 19 as an iron item (DMG ibid), how should the arcane lock make them more difficult to damage; by increasing their AC or hp, or both? What increase would be equivalent to a +10 increase to the DC of a strength check?
(1) The players have just started Rise of Tiamat but have not played Hoard of the Dragon Queen. I was using the adventure hook
wherein a captive dragon cultist is being escorted to the alliance council meeting but an ambush is made to attempt to free or kill her.