MCB Types for hospitals

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Hi guy and gals, Im an electrician that has started in a hospital and have been tasked with changing a distribution board.
Thry are currently old type 3 3871 mcb, and I was going to change them for 32 Amp rcbo type c rated, 30mA . The circuites are mainly ring mains, that cover patient rooms.
The equipment plugged in would be beds with motors, medical equipment with pumps, and other portable appliances. I am worried as the ZS values of the circuits are higher than the MCB values. Or would the Rcd cover this ?
Any thoughts would be great.
 
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1) You shouldn't rely on the RCD part to cover for otherwise poor design, a circuit on a TN should always meet discon. times for the magnetic part of the breaker, IMHO

2)Choose type: B, C, D to suit inrush of load while keeping an eye on max permitted zs, don't use a less sensitive one if theres no need... eg, you socket below the DB might be low enough for a type D, but that doesn't mean you should fit one...

3) Look up the Meigan code of practice
 
1) You shouldn't rely on the RCD part to cover for otherwise poor design, a circuit on a TN should always meet discon. times for the magnetic part of the breaker, IMHO

2)Choose type: B, C, D to suit inrush of load while keeping an eye on max permitted zs, don't use a less sensitive one if theres no need... eg, you socket below the DB might be low enough for a type D, but that doesn't mean you should fit one...

3) Look up the Meigan code of practice

Thanks for the reply, what im trying to make sure doesnt happen is the RCBO tripping when medical equipment is being used. This is just for the ward area and patient rooms. Some of the existing runs are very long and the existing breakers are type3.
I was just concearned that the ZS values are higher than the breaker values. This is an old installation im working on and not new, otherwise I could redesign the whole circuit.
 
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Never, ever, ever use an RCD to paper over the cracks of too high a Zs in a TN installation.

Never.
 
You should know this if you are doing THIS job.

Try loooking at 411.4.9

but note

415.1
Thanks for the reply, Im working with an old installation and trying to minimize nuisance tripping. I am new to this area, so that is why im asking. I cant know everything. The existing board they are using type 3 breakers, and type C are generally used for commercial situations, to cover light motors, pumps anything with charging facilities. So type C fits this catagory, but I was concerned about the ZS values, as they are long runs.
Its much easier to design from new, but im working with old.
 
Some of the existing runs are very long and the existing breakers are type3.
I was just concearned that the ZS values are higher than the breaker values.
Are you saying that you know the Zs values are too high for disconnection (in which case I would have thought that you would be obliged to advise that re-wiring is really required) or is it just that you fear that the Zs values may be too high?

(and perhaps I should mention that I once saw a motorised hospital bed, complete with patient in it, burst into flames!)

Kind Regards, John.
 
Some of the existing runs are very long and the existing breakers are type3.
I was just concearned that the ZS values are higher than the breaker values.
Are you saying that you know the Zs values are too high for disconnection (in which case I would have thought that you would be obliged to advise that re-wiring is really required) or is it just that you fear that the Zs values may be too high?

(and perhaps I should mention that I once saw a motorised hospital bed, complete with patient in it, burst into flames!)

Kind Regards, John.

Hi John,

I dont have the results with me, but i belive they are higher that what a type C would allow, so i will have to look at Type B.
It is a 4mm ring main in steel trunking and conduit. They have had contractors in the past doing room refurbishment and electrical alterations
, I have tested the board, and recomended a change of board, as the existing one is original, and spare parts are hard to get hold of. Along with sockets needing 30 mA and any new cables being installed with it. Not using type C before, I was surprised at how low the reading are.
Just want to get the job right on the first board.
Many thanks
 
I dont have the results with me, but i belive they are higher that what a type C would allow, so i will have to look at Type B.
This is beyond my experience and comfort zone, but I'm sure others will be able to help. In my innocence, I would have thought that the type of device you use (B or C) would be dictated by the nature of the load, and that if the load was such as to require Type C, you'd be at risk of just be back to nuisance tripping if you used Type B to overcome a problem of Zs being too high. Mind you, I guiess you also ought to find out the Zs at the board - I suppose it could be sufficiently hgh that no sensible size of cable for the RFC is going to get you an acceptable Zs for a Type C device, in which case I don't know what you'd do - revert to the not-very-nice solution of relying on the RCD?

Kind Regards, John.
 
Hospitals are a special case. The new amendment it seems will have a special section for hospitals. You can get 13A plugs without a fuse (red in colour) special for hospital equipment.

As a result I would never design for a hospital as there are so many special rules.

I would expect it would require special consultation and agreement and possible use of this
type of RCD which auto resets or even an RCM instead of RCD. But it is a specialised field and if you don't have the specialised knowledge then don't tender for the work.
 
I dont have the results with me, but i belive they are higher that what a type C would allow, so i will have to look at Type B.
This is beyond my experience and comfort zone, but I'm sure others will be able to help. In my innocence, I would have thought that the type of device you use (B or C) would be dictated by the nature of the load, and that if the load was such as to require Type C, you'd be at risk of just be back to nuisance tripping if you used Type B to overcome a problem of Zs being too high. Mind you, I guiess you also ought to find out the Zs at the board - I suppose it could be sufficiently hgh that no sensible size of cable for the RFC is going to get you an acceptable Zs for a Type C device, in which case I don't know what you'd do - revert to the not-very-nice solution of relying on the RCD?

Kind Regards, John.

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.
Just thought I would share the latest with you.

Hi John,

I have recently checked the results, and they all come under the 0.57 ZS values. It
 
If you suspect circuit lengths are on the long side you will need to check for short circuit protection as well (L to N) which an RCD will not protect for anyway. You do not want excessively long faults on cables causing thermal damage.
 

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