What is "low enough" rather depends on the credible fault current, and that you can probably estimate from the current rating of the equipment in question and then a large factor for safety perhaps ten, so  say you had a 32 A supply, assume you' like an earth path that can take 320A or more to operate a breaker smartly, so your half ohm figure is about right, but on the other hand, for a thing enclosing a 200A supply it may not be good enough.

Arguably bits of the machine that do not come anywhere near any wiring, and there is no credible fault path that would make it live, are less important - so small access panels to lubrication points or something that are nowhere near any wiring, are not really worth worrying about.

Mike.

CarloUK said:

I want to regularly check that all sections of the machine is earthed/bonded to the machines main earth incoming cable.

How is this done. Do we just resistance check from the incoming earth to each section. If so what is an acceptable resistance. ie Less than 0.5 ohms ?

I would recommend that safety checks of machinery are only carried out by competent persons who know:
 - the relevant product safety standards
 - the particular requirements and recommendations of the manufacturer of the machinery.

If we are talking about Machinery to BS EN 60204-1, the correct test of the protective bonding circuit I believe you may be after is described in BS EN 60204-1 (although if there is a dedicated product standard for the particular machine that should be consulted):

18.2.2 Test 1 – Verification of the continuity of the protective bonding circuit

The resistance between the PE terminal (see 5.2 and Figure 4) and relevant points that are part of the protective bonding circuit shall be measured with a current between at least 0,2 A and approximately 10 A derived from an electrically separated supply source (for example SELV, see 414 of IEC 60364-4-41:2005) having a maximum no-load voltage of 24 V AC or DC.

The resistance measured shall be in the expected range according to the length, the cross sectional area and the material of the related protective conductors and protective bonding conductor(s).

Earthed PELV supplies can produce misleading results in this test and therefore shall not be used.

NOTE Larger currents used for the continuity test increases the accuracy of the test result, especially with low resistance values, i.e. larger cross sectional areas and/or lower conductor lengths.

The manufacturer should provide guidance on procedures for safe maintenance of a particular machine, and you should take these into account to avoid damage to electronic circuits, in particular interlocks and safeguards.

mapj1 said:

Arguably bits of the machine that do not come anywhere near any wiring, and there is no credible fault path that would make it live, are less important - so small access panels to lubrication points or something that are nowhere near any wiring, are not really worth worrying about.

Whilst this would be a generally the case for many products, it is not necessarily to the case where the parts are required to be connected to the protective bonding circuit of machinery according to BS EN 60204-1.