MCB Types for hospitals

[holmslaw]

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[thomo27]

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[Adam_151]

thomo27 wrote:

Also the steel conduit is used as the CPC

Not in the actual medical areas, I hope... well at least not in recently refurbished areas at any rate

[holmslaw]

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[RF Lighting]

Holmslaw is spot on

I would have thought you would want to avoid the use of RCDs in a hospital environment as much as possible.

But then again I don't know much about these special installations, and would leave it to someone who does to at least spec the job.

It's frightening that you are asking for advice on how to design an installation in such a safety critical environment where people could die if you get the design wrong on a DIY website

[Adam_151]

holmslaw wrote:

Nothing wrong with steel conduit/trunking as cpc.

Generally no, but not for medical areas to the MEIGaN guidence...a lot of which is about maintaining a low impedance earth path between metalwork within the medical location, etc... I presume the worry against using metallic contaiment as the earth is that... particualy in areas which are cleaned regulaly, corrosion could creep into the joints. The other issue is parallel paths, quite often the equipment needs a clean earth, which requires a separate earth conductor, rather than use the containment which might introduce electrical noise due to protective conductor currents from other equipment, etc, etc

[ricicle]

Most critical (theatre) areas are designed as isolated areas and so the introduction of an extraneous earth via steel conduit is a no no !!!!
These areas are beyond the scope of BS7671 and a DIY site !

[JohnW2]

steveo12350 wrote:

Hi John, I have checked the values, and they all come under the 0.57 ZS values. So this seems to be covered. Im thinking type C still the best, as patients are often hooked up to drip machines.

Thanks for the update - but I see that some 'debate' continues!

As for your understandable concerns about 'drip machines', a lot of infusion pumps, syringe drivers and drip counters/regulators (as well as otheer crucial equipment) have built in battery back-up - although I obviously don't know what equipment may be in use at your site.

Kind Regards, John.

[flameport]

steveo12350 wrote:

Hi guy and gals, Im an electrician that has started in a hospital.

This is NOT meant as any criticism of your abilities, but please explain how you are tasked with replacing a distribution board in a hospital yet apparently have no experience of working in such places?

[securespark]

If your sockets are fed via surface-mounted cabling and/or earthed conduit, then you could do away with RCBO protection on the sockets dedicated to equipment.

The sockets would need to be labelled as such and/or the type with a non standard plug/socket arrangement.

Type C Zs max. values approx half that of Type B's. And type D's half that of Type C's.

Oh, and an addition to the conduit as cpc debate. Yes, it's fine, as long as all the joints are tickety-boo. I'd be using a high current clair tester to confirm this. Not that they are de rigueur anymore. It seems the inclusion of a cpc in metal conduit systems has made them obsolete.

[JohnD]

some hospital RCBOs are 10ma, since the risk of shock when you have equipment attached to, or inserted in, your body or its fluids is particularly high.

I too have no experience of the standards, and I am bemused that you have not been given a specification to work to, drawn up by someone who knows.

[JohnW2]

JohnD wrote:

some hospital RCBOs are 10ma, since the risk of shock when you have equipment attached to, or inserted in, your body or its fluids is particularly high.

That is true, but it's always worried me a little - since it could be taken as an invocation of a variant of the misconception I've been discussing here over the weekend - in this case that a 10mA RCD/RCBO will result in a lower current flowing through a victim (under leakage conditions) than would a 30mA one.

If it's a gradually developing fault/leakage (with gradually increasing leakage current), then the 10mA device obviously does provide additional safety, but faults tend to arise suddenly.

Kind Regards, John

[JohnD]

As I recall, an RCD can trip as soon as the sine-wave of an AC supply rises to a voltage where the current flowing reaches its trip point. This iks why, with a fairly significant current, they can trip within less than half a cycle (1/100th of a second at 50Hz)

Death and injury from electric shock are a function of current x time, so the 30mA RCD is chosen as it will give a good chance of survival and a very good chance of avoiding soft tissue or bone damage which can be quite horrible otherwise.

[JohnW2]

JohnD wrote:

As I recall, an RCD can trip as soon as the sine-wave of an AC supply rises to a voltage where the current flowing reaches its trip point. This iks why, with a fairly significant current, they can trip within less than half a cycle (1/100th of a second at 50Hz)

One might think so, and it may be true on the bench with large current imbalances (i.e. very high fault currents to earth), but we know from RCD testing and test result requirements that RCDs don't operate in less than quarter** of a cycle (5msec) in response to moderate fault currents. Perhaps it's true that they do trip in an electrical sense within that period, but then take appreciably longer than that for the mechanical disconnection to occur.

{** I say quarter of a cycle, rather than the half-cycle you mention, since the voltage is only rising for the first half of the first half cycle}

I understand even less what is going on with fault currents around the rated trip level of the RCD. In such cases, disconnection can take many cycles - and I'm not sure why, if it doesn't trip during the first quarter cycle, it is ever going to trip subsequently. Something 'cumulative' is presumably going on, but I can't think what. Anyone?

Provided it does not result in nuisance tripping, using a 10mA RCD rather than a 30mA one can't do any harm, and may indeed result in lower current*time prior to disconnection - so it's certainly a good idea to have it in situations such as you mentioned. However, without understanding a lot more about details of the operation of the devices I would be far from confident that the difference would necessarily be anything like as great as one might expect.

Kind Regards, John.

[ban-all-sheds]

JohnW2 wrote:

{** I say quarter of a cycle, rather than the half-cycle you mention, since the voltage is only rising for the first half of the first half cycle}

It's also rising for the 3rd quarter of the first cycle.