Rcd's in series..

[apprentice87]

Hi Graham,

Rcd's are terribly unreliable, I read somewhere of a 7% failure rate. Think your car would be allowed on the road if that type of car had a brake failure rate of 7%....

As on a TT system, unless Zs is helped by bonding to extraneous parts, in the event of an earth fault, there will be no ADS, [unless you have a small MCB and a mega low Ra] so all exposed parts will be permanently live, and the first person to touch one will die..

Why not three MCB's in series?? Simple, because an MCB is there merely to protect conductors against overcurrent, not to act as "additional protection" against shock hazards and/or to achieve required disconnection times in the event of earth faults...

An MCB fails, yes, insulation could be damaged, and this could indeed cause an earth fault and make exposed parts live, but then an upstream RCD in a modern system will clear the fault...or at least you hope it will, cos if you touch the live exposed part, the RCD will be your last resort..

john....

 

[Sidewinder]

Graham,

The jobs are medical where there are applied parts.

The upstream RCD device could well be at the origin of the circuit.

Property of the patient, or their landlord.

Where we make our connections to the existing circuit for the medical equipment we fit an RCD, this is the "property" of the Health Trust.

At the connection of the medical equipment to the final circuit, we fit an RCD, manufacturers instructions.

These devices are tested at around 6 month intervals at the longest, even though the patient is advised to test them between this time.

They almost certainly do not.

Where there is an "up front" RCD we will advise testing of this quarterly, however, neither myself or the Trust have control over any up front device.

As RCD's are statistically the most unreliable form of protection and being used as additional protection then a design decision was to put 2 series RCD's into "our" systems under control of the Trust that can be tested independent of the rest of the install.

Also in the event of an actuation in anger as it were, we feel we have a better chance of at least one of "our" devices operating.

This will hopefully keep the patient alive and will allow power to remain on in the rest of the premises.

We all realise that there is no true discrimination, however, this madness as you term it was felt justified in an attempt to ensure patient safety was of the highest level whilst they are under treatment.

The other option we have currently is to turn the area of the premises being used into a Group 2 Medical Location, see section 710, BS7671:2008 Amd 1:2011, the cost would be exorbitant, and the complexity of doing this in the environment would be highly disruptive.

 

[Technician]

Redundancy; a word that originated IIRC from within the building industry

that meant the inclusion of structural features that could be lost, but the

structure as a whole would remain unaffected.

Subsequently used to describe the process whereby unnecessary staff and

employees were "outsourced" from organisations so that pension obligations

could be avoided.

Now built into electrical systems to ensure that additional protection is assured

when there is a statistical probability that one device (on its own) cannot be

depended upon to operate within the required disconnection times.

Looks like madness, but probably the only way out. Very interesting comments.

 

[ProDave]

The jobs are medical where there are applied parts.The upstream RCD device could well be at the origin of the circuit.

Property of the patient, or their landlord.

One of the reasons given in the OP for fitting 2 rcd's was they are unlikely to be tested ever, so more likely not to work when needed.

I find that statement that they are unlikely to ever be tested completely unacceptable in a medical environment. Surely testing there should be far more stringent and regular than a domestic situation, where I agree the test button never ever gets pressed.

 

[Chr!s]

Graham,The jobs are medical where there are applied parts.

The upstream RCD device could well be at the origin of the circuit.

Property of the patient, or their landlord.

Where we make our connections to the existing circuit for the medical equipment we fit an RCD, this is the "property" of the Health Trust.

At the connection of the medical equipment to the final circuit, we fit an RCD, manufacturers instructions.

These devices are tested at around 6 month intervals at the longest, even though the patient is advised to test them between this time.

They almost certainly do not.

Where there is an "up front" RCD we will advise testing of this quarterly, however, neither myself or the Trust have control over any up front device.

As RCD's are statistically the most unreliable form of protection and being used as additional protection then a design decision was to put 2 series RCD's into "our" systems under control of the Trust that can be tested independent of the rest of the install.

Also in the event of an actuation in anger as it were, we feel we have a better chance of at least one of "our" devices operating.

This will hopefully keep the patient alive and will allow power to remain on in the rest of the premises.

We all realise that there is no true discrimination, however, this madness as you term it was felt justified in an attempt to ensure patient safety was of the highest level whilst they are under treatment.

The other option we have currently is to turn the area of the premises being used into a Group 2 Medical Location, see section 710, BS7671:2008 Amd 1:2011, the cost would be exorbitant, and the complexity of doing this in the environment would be highly disruptive.

Are you saying the installations comes under Group 2 Medical Location?

If so are you departing from BS7671 and suggesting your system as an alternative?

 

[Chr!s]

Please explain what you mean by strapping the RCD?

Which conductors do you place your leads on to conduct the test?

 

[Sidewinder]

One of the reasons given in the OP for fitting 2 rcd's was they are unlikely to be tested ever, so more likely not to work when needed.I find that statement that they are unlikely to ever be tested completely unacceptable in a medical environment. Surely testing there should be far more stringent and regular than a domestic situation, where I agree the test button never ever gets pressed.

It is unacceptable in a medical environment that the devices are not tested, hence why we install our own devices so that they can be tested.

How would you feel if you had no understanding of electrical "stuff" and someone came along every 3 months and turned your electric off and back on for your house?

---------- Post Auto-Merged at 19:16 ---------- Previous post was made at 19:12 ----------

Are you saying the installations comes under Group 2 Medical Location?If so are you departing from BS7671 and suggesting your system as an alternative?

Yes I am, and neither the IET or the NICEIC Tech bods, have come up with an alternative that is workable, so I will stand my ground all the way to the EU court if I have to.

The Regs committee did not envisage the installs that I do when they wrote 710, this is patently obvious otherwise I would have had a response by now to the communications I sent.

I have not.

I will carry on the way I am doing until the IET come up with a workable alternative end of.

They will not, in fact neither have several other extremely competent, experienced and qualified electrical engineers.

---------- Post Auto-Merged at 19:17 ---------- Previous post was made at 19:16 ----------

Which conductors do you place your leads on to conduct the test?

Hang fire, YOU first, YOU tell me, what YOU describe as strapping.

Then we can move forward.

 

[ProDave]

How would you feel if you had no understanding of electrical "stuff" and someone came along every 3 months and turned your electric off and back on for your house?

As long as they told me they were turning it off for an essential safety check it would be fine.

Most people don't understand gas, but do understand a gas boiler needs an annual service and inspection to remain safe.

I find it inexcusable to hear that RCD's in a medical environment don't have a regular test routine. What's the normal PIR interval in a hospital? oh don't tell me they don't do those either, just fix things when they break?

 

[Sidewinder]

Dave,

WHO said this was IN a Hospital!

 

[davetheglitz]

Reply from ProDave No, what the OP is talking about is two RCD's in series. And as above, both will see the fault current and both will trip. That's precicely what the OP is wanting to achieve, more chance of a trip in the event of a fault.

You're absolutely right - I had a whisky induced mental blank!!!

 

[Chr!s]

It is unacceptable in a medical environment that the devices are not tested, hence why we install our own devices so that they can be tested.How would you feel if you had no understanding of electrical "stuff" and someone came along every 3 months and turned your electric off and back on for your house?

---------- Post Auto-Merged at 19:16 ---------- Previous post was made at 19:12 ----------

Yes I am, and neither the IET or the NICEIC Tech bods, have come up with an alternative that is workable, so I will stand my ground all the way to the EU court if I have to.

The Regs committee did not envisage the installs that I do when they wrote 710, this is patently obvious otherwise I would have had a response by now to the communications I sent.

I have not.

I will carry on the way I am doing until the IET come up with a workable alternative end of.

They will not, in fact neither have several other extremely competent, experienced and qualified electrical engineers.

---------- Post Auto-Merged at 19:17 ---------- Previous post was made at 19:16 ----------

Hang fire, YOU first, YOU tell me, what YOU describe as strapping.

Then we can move forward.

I can see your predicament, though would you not be better using isolation?

 

[Chr!s]

Are you sure it's class 2?

 

[Sidewinder]

Are you sure it's class 2?

Is what class 2?

Not sure what you are referring to?

:Salute

 

[sparkytim]

Because when you run an RCD test on the second one, both will almost certainly trip, so how do you know if the trip time you measure is the first or second one tripping?Or am I being the one a bit thick here?

Trick to testing RCDs in series.

Upstream RCD you can test at the RCD with no problem

Downstream Rcd Test between Line Terminal on LOAD SIDE and N term on Supply side.

This will trip THIS Rcd only as the Up stream RCD does not see an imbalance.

 

[steptoe]

Trick to testing RCDs in series.Upstream RCD you can test at the RCD with no problem

Downstream Rcd Test between Line Terminal on LOAD SIDE and N term on Supply side.

This will trip THIS Rcd only as the Up stream RCD does not see an imbalance.

really?

considering that you are leaking current to earth I find that very hard to understand,

and Im not convinced.

actually,

I think that is totally wrong, there is no way that is a given,

its simply not correct,

I suggest you draw it out and add in the leakage paths and you will understand how flawed that suggestion actually is.

 

[Chr!s]

Is what class 2?Not sure what you are referring to?

:Salute

Sorry Paul

The other option we have currently is to turn the area of the premises being used into a Group 2 Medical Location, see section 710, BS7671:2008 Amd 1:2011, the cost would be exorbitant, and the complexity of doing this in the environment would be highly disruptive.

Group 2, or Clinical Risk Cat 4,5. Has a risk assessment really shown this to be the case?

 

[sparkytim]

really?considering that you are leaking current to earth I find that very hard to understand,

and Im not convinced.

actually,

I think that is totally wrong, there is no way that is a given,

its simply not correct,

I suggest you draw it out and add in the leakage paths and you will understand how flawed that suggestion actually is.

Are but you are not leaking current to earth but to the Nuetral

Try it and see!

 

[Chr!s]

really?considering that you are leaking current to earth I find that very hard to understand,

and Im not convinced.

actually,

I think that is totally wrong, there is no way that is a given,

its simply not correct,

I suggest you draw it out and add in the leakage paths and you will understand how flawed that suggestion actually is.

Steps, this is called strapping the RCD, you will find reference to it in later versions of Fluke and Megger manuals. Your testing the RCD, not the connected system.

This is also the method to use where a high Ra may be present.

You may find this in the new GN3, as they are aware of the 50V requirement placed on the test equipment manufactures. If you have a Ra greater than 333 ohms it will not test, yet a Ra 0f 333 ohms is allowed by BS7671, so the only way to test is to strap. The Manufacturers of the RCDs are also happy with this method.

 

[steptoe]

Steps, this is called strapping the RCD, you will find reference to it in later versions of Fluke and Megger manuals. Your testing the RCD, not the connected system.This is also the method to use where a high Ra may be present.

You may find this in the new GN3, as they are aware of the 50V requirement placed on the test equipment manufactures. If you have a Ra greater than 333 ohms it will not test, yet a Ra 0f 333 ohms is allowed by BS7671, so the only way to test is to strap. The Manufacturers of the RCDs are also happy with this method.

seriously?!

thats madness,

so you are testing the effectiveness of a device that measures an leakage to earth and you dont use the earth to test it with?

what about ELCBs?

you may as well just short out L load with N supply using a bit of wire,

IMHO that test isnt worth much more than pushing the test button,

 

[sparkytim]

The test button only uses a fixed resister to test function. RCD tests introduce current at set levels and record the time taken to trip. This is not an Earth test. Thats a seperate Ra Test for TT or Ze for TN