In my view, when the district heating pipes enter the building they should be bonded to the MET, just as one should bond other incoming metallic services.

The external pipes, buried outside, do not need bonding any more than water mains in the street need bonding.

Extract from 411.3.1.2 (and as stated by Broadgage)

In each installation main protective bonding conductors complying with Chapter 54 shall connect to the main earthing terminal extraneous conductive-parts including the following:

(i) Water installation pipes
(ii) Gas installation pipes
(iii) Other installation pipework and ducting
(iv) Central heating and air conditioning systems
(v) Exposed metallic structural parts of the building.

However there is a caveat:

'Metallic pipes entering the building having an insulating section at their point of entry need not be connected to the protective equipotential bonding.'

Is the pipework really metallic underground? Rigid pipework would be a pain to feed through ducts (presumably with 90 degree bends to arrive out of the ground vertically) - and all the pre-insulated tubing I've looked at has been plastic (usually PEX) which is much more flexible.

Could it be what we're looking at is a transition from plastic to copper, that's then covered with thermal insulation?

Test wise, the old school method would be to test against a temporary electrode (say a long screwdriver shoved into the mud outside) - even if it's off by a few hundred or even a thousand Ohms or so, it should still give you a sensible ball park reading, since you're looking for tens or hundreds of k Ohms.

   - Andy.

Hi Toby. It appears, in this case not currently possible to determine if an extraneous-conductive-part through measurement? The image appears to depict a conductive component that could introduce a potential and is not part of the electrical installation. Just to add, it is important to consider that even an apparent extraneous-conductive-part, which is not liable to be touched, would not necessitate bonding. For instance, this applies to ‘earthy’ metallic pipework that is enclosed by insulating material (e.g., boxed in).

Or another thought on testing - with a long lead could you do a continuity test between the pipes in two different places (e.g. next door buildings)? That might actually be a better test anyway, since the thermal insulation may electrically insulate the pipes from Earth, but wouldn't prevent potentials being transferred from one installation to another along the pipes (if they are metallic).

   - Andy.

seconded. Plant a real extraneous conductive part in some real mud, and measure the resistance  to it - no need for power or a MET.

Biggest risk of bonding here is that you are introducing links between the neutrals of substations that would normally never meet - 1km is a 'long way' in LV land. During thunderstorms and so on very large voltages may be induced as well as the risk of very significant diverted neutral currents in normal operation.

Even if test show it is an extraneous conducting part,  serious consideration should be given to the use of insulating pipe sections and enclosures for the bit of visible metal as a safer  alternative to the normal 'bond everything in sight'. That might depend on the circulating fluid and its conductivity however.
Mike.

Hi, Thankyou for the responses, I did consider this however at points the metallic pipe work is exposed for gate valves ect. If it was enclosed in its entirety then sure I would understand why this might negate bonding however these exposed parts are liable to be touched. 

Hi, Thanks for the reply. Yes the pipe work is fully metallic, Its installed in trenches in the early construction phase and welded. You're right I shouldn't be that concerned about a few ohms because of a poor earth. I took your advise and used a long lead on multiple exposed parts from this building and buildings nearby e.g Cranes, Temporary power panels, MET next door. I got a mix of readings from 0.05mo to 0.02mo. 

Hi Toby,
I wanted to inquire if the gate valves etc are simultaneously accessible to exposed conductive parts.

Hi 

Yes, The pipework runs parrel to a ELV tray for sensors which leads to the BMS panel, There's also serval FCU above the pipe work for Heat meters ect.