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BS 7671 interpretation of clause 560.7.7

Within BS7671 there is the following clause:-

560.7.7 Safety circuit cables, other than metallic screened, fire-resistant cables, shall be adequately and reliably separated by distance or by barriers from other circuit cables, including other safety circuit cables.

Now I read this in two ways, the first is this is specifically talking about fire rated cables only… or

This is insinuating that unless I use a metallic screened fire-resistant cable I must provide separation by distance or by barriers including from other safety circuit cables. Does this mean a common dedicated medical IT final circuit trunking is not acceptable as each IT circuit in its own right is a safety circuit, therefore if I was to move away from fire rated cable I would need to ensure each IT circuit is installed in its own individual piece of containment (conduit) to give the correct level of separation or would a common galvanised trunking dedicated to the IT cabling be sufficient.

I appreciate no one will give me an answer to the question above, but can people let me know how they interpret this clause to see if I’m on the right lines.

Can I ask how do you interpret this clause?

0 Roy Bowdler over 5 years ago

I recall a situation some years ago in which Bed Head Trunking produced by a particular supplier had been specified by the designer, on the basis that it had been “approved as a medical device”. Our design department pointed out that other supplier’s containment was substantially the same, but available at much lower cost. I can’t recall the overall cost difference, but it would have been 100K plus. So we have an ambiguous situation involving several powerful stakeholders, with one or more potentially standing to lose money.

Graham makes the point well and illustrates that engineering judgments are often about coming to an optimum solution (aka compromise) not theoretical “perfection”. I also don’t recall the outcome, although readers familiar with current policy around HTM 08-03 may be more familiar.

When I was first trained around the 14^th Edition (1974 amendments), the limitations of the scope of the regulations was emphasised (I was in a Power Station Apprentice) and that they formed part of a raft of relevant laws, codes and standards. The (CEng) trainer also emphasised in his words "that they did not preclude any other methods determined by a suitably qualified electrical engineer”. I don’t know if the phrasing was his own, or a quotation?

When some years later I was responsible, an “Electricity Supply Industry Electrician’s Guide” was available, which covered most common situations to the extent that a Electrician/Technician/Engineer not primarily responsible for significant design would usually need. With the modern emphasis on risk management and a more litigious environment, these issues can be complex and require significant expert judgement/control. My example also illustrates how responsibility has been fragmented by the tendency to compartmentalise and sub-contract. This “shares” risk, but requires considerably more than pure technical skills to manage.

Perhaps this anecdote helps? One of the UKs leading Leadership Training organisations, has its roots in “outdoor development” carried out in The English Lake District. Its first three significant customers were Post Office Telecom (now BT), The National Coal Board (no successor) and the Haden Group (absorbed into Balfour Beatty). Presented with similar challenges, each illustrated a different approach. One would acquire every possible bit of equipment, ie “throw money at it”, another would use their collective physical strength if possible and the third would nominate a representative to negotiate with those managing the challenge.
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0 euringharris over 5 years ago

Alasdair Anderson:

This is to do with circuits for Safety Services, defined as:
Safety service. An electrical system for electrical equipment provided to protect or warn persons in the event of a hazard, or essential to their evacuation from a location.

Based on this I would not classify dedicated medical IT final circuit cabling as a safety service and so this requirement would not apply.

Others may have different opinions of course.

Alasdair

Hi Alasdair

I will try to provide an abridge response

Whilst noting your definition quote the hazard in the instance of death through failure of the supply (part of the Group 2 definition). Furthermore the examples in 560.1 (and I know they are notes) states essential medical systems along with industrial systems etc. So goes beyond the world of fire fighters lifts fire alarm supplies etc.

In terms of Section 710 there are numerous references to connection to electrical supply system for safety services or similar. Looking at this if the load needs to be connected to the safety supply then it will be part of that same system and the cable or cables that connects a load requiring a safety service supply will need to meet the requirement for a safety circuit.

This thread is looking at fire resistance but there are lots of other requirements for safety circuits.

As hospitals employ progressive horizontal evacuation (PHE) this means that there is the possibility that the patient end of the service may be in a separate compartment ( or sub-compartment or department) than the fire and as such is held. If the cable or cables supplying the patient pass through a sub-compartment which may be in fire then it would be reasonable to expect that essential power remain on as long as the patient is expected to be held.

Therefore, this resistance to the effects of fire can be achieved by a number of methods (too much for this thread). as long as Section 710 and all the other general parts of BS 7671 have been satisfied.

Linking this back to the guidance in HTM 06-01, this follows on from lots of other HTM guidance regarding separating essential circuits or where not feasible using fire resistant cabling so it offers an easy fool proof solution suggesting fire resistant cabling but if you are able to do it by another no less safe way then do that- This approach is applicable because of Clause 3.26 of HTM 06-01 which was originally based on the intended departure clause of BS 7671 120.3.

Paul
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0 euringharris over 5 years ago

Roy Bowdler:
I recall a situation some years ago in which Bed Head Trunking produced by a particular supplier had been specified by the designer, on the basis that it had been “approved as a medical device”. Our design department pointed out that other supplier’s containment was substantially the same, but available at much lower cost. I can’t recall the overall cost difference, but it would have been 100K plus. So we have an ambiguous situation involving several powerful stakeholders, with one or more potentially standing to lose money.

Graham makes the point well and illustrates that engineering judgments are often about coming to an optimum solution (aka compromise) not theoretical “perfection”. I also don’t recall the outcome, although readers familiar with current policy around HTM 08-03 may be more familiar.

Hi Roy

I am not a great fan of manufacturers who capture a market and then capitalise on this trying to exploit the sector . However, what is often the case is that contractors often see specific pieces of equipment as standard commodities such as cable tray or trunking. I refer to that by your reference to 'containment'.

Medical Trunking systems are not medical devices (covered by the BS EN 60601 series of documents) but instead is covered by the Medical Supply Unit standard BS ISO 11197 and as such are slightly more than a piece of containment in fact in most instances if the installer uses the trunking as containment by adding cabling that is not intended then it will probably invalidate any conformity certification.

However, if your £100k was for another BS ISO 11197 compliant system then that is a different matter.

Ultimately, the responsibility for compliance with Standards rests with the designer under Regulations 511.1 and 511.2 and the designer is correct to specify the equipment to the correct standard.

regards

Paul
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0 euringharris over 5 years ago

I think in reality this this needs addressing in the next version of the HTM otherwise Contractors will continue to be scared of non-complying to the regulations unless they use fire rated cabling.

Hi Matt

Adding a UPS increases the fire risk so that in itself needs further consideration - but regardless of that the guidance is there to make a fool proof solution (based on Firecode requirements from the 1990s through to today) Obviously lots of electrical designers have not read HTM 81 or any of its successors or they would be aware of this.

If you don't want to use fire resistant cabling and you have sufficient mitigating measures to provide an alternative solution you can follow the procedure in Clause 3.26 of the HTM, However, you will need to get that departure agreed (and take responsibility for that departure if it turns out not to be correct)

.....Any proposed departures, including any equivalent or mitigating measures to be applied, should be discussed with the relevant stakeholders

including the Electrical Safety Group....

If it doesn't stack up when you present your risk assessment (ensuring all risks including the addition of the UPS to the space which will more than likely change the space assessment) then you will need to apply further mitigation or revert back to guidance.

regards

Paul
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BS 7671 interpretation of clause 560.7.7

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