Greetings all,
In a commercial three phase installation, there are often (almost always) single phase 10 A switched socket outlets (power points) on different phases (some on red, some on white and some on blue). It has recently been suggested to me that when a meeting room (office type of environment) with outlets on red phase, is opened up (folding divider wall/door scenario) to link with another room with outlets on white phase for example, a non-compliant situation is created because there are power points on different phases, in the same room. While I do realise that two faulty class one appliances (open circuit earth and an earth fault on each) could create a situation where there is 400V between the metalwork of the appliances, I am not aware of any legislation that makes this situation illegal (non compliant). Does anyone have any references from Electricity Regulations or Standards Cited within the Regulations, that addresses this situation in support of the claim that this is illegal?
Thanks
Gaz
Switch Socket Outlets on different phases in same room
Re: Switch Socket Outlets on different phases in same room
There isn't any.
And the scenario proposed, of a hazard arising from two Class I single phase appliances plugged into different phases , and with simultaneous earth fault isn't valid. Assuming the normal fault protection by automatic disconnection, then for each appliance the disconnection occurs within 0.4 sec.
Whoever suggested that it's a compliance issue is just dreaming.
If it was a problem to have single-phase sockets in same general area on different phases; logically multi-phase sockets would have to be banned.
In fact multi-phase equipment generally would have to be banned, or at least not simultaneously accessible; max disconnection time of 5 sec clearly creating more opportunity for such mishap than for sockets with 0.4 sec.
Fact is Wiring Rules don't regulate on the basis of multiple simultaneous faults.
And the scenario proposed, of a hazard arising from two Class I single phase appliances plugged into different phases , and with simultaneous earth fault isn't valid. Assuming the normal fault protection by automatic disconnection, then for each appliance the disconnection occurs within 0.4 sec.
Whoever suggested that it's a compliance issue is just dreaming.
If it was a problem to have single-phase sockets in same general area on different phases; logically multi-phase sockets would have to be banned.
In fact multi-phase equipment generally would have to be banned, or at least not simultaneously accessible; max disconnection time of 5 sec clearly creating more opportunity for such mishap than for sockets with 0.4 sec.
Fact is Wiring Rules don't regulate on the basis of multiple simultaneous faults.
Re: Switch Socket Outlets on different phases in same room
Thanks AlecK,
This is what I also thought, checking all the documentation I had and after many years of encountering situations just ike this.
Just to clarify though, out of interest, the scenario proposed to me was open circuit earth on each appliance AND a fault phase to frame on each appliance so the fault on each is phase to frame, and the frame is not earthed, so there will be no tripping of MCB or blowing of fuse. The metal frame of appliance 1 being alive at phase potential (RED) and the frame of appliance 2 being alive at phase potential (WHITE). The potential between appliance frames being 400V.
Thanks and cheers
Gaz
This is what I also thought, checking all the documentation I had and after many years of encountering situations just ike this.
Just to clarify though, out of interest, the scenario proposed to me was open circuit earth on each appliance AND a fault phase to frame on each appliance so the fault on each is phase to frame, and the frame is not earthed, so there will be no tripping of MCB or blowing of fuse. The metal frame of appliance 1 being alive at phase potential (RED) and the frame of appliance 2 being alive at phase potential (WHITE). The potential between appliance frames being 400V.
Thanks and cheers
Gaz
Re: Switch Socket Outlets on different phases in same room
- earth fault on item 1
- open-circuit PEC in item 1
- earth fault on item 2
- open-circuit PEC in item 2
That's not just 4 simultaneous faults; but a particular combination of faults.
Plus a fifth condition: that the two items are simultaneously accessible.
It's true this scenario could result in a hazardous voltage between two simultaneously accessible items;
and the suggestion was not just that there should be a rule to avoid this, but that there already is one.
Which demonstrates a lack of joined-up thinking.
The point of having a set of rules for installation wiring is to manage the risks of using electricity.
It's simply not possible to make it risk free; so we aim for an acceptable level of risk - at an acceptable cost.
(And in setting that level; the risks & costs of not using electricity for the task have to be considered as well).
Risk management 101 says that risk is a function of the likelihood of an event combined with effect if it were to happen.
Before making rules to avoid a very unlikely scenario; logically we'd have to have rules that eliminated all the more likely scenarios that could result in a similar outcome.
Looking at outcomes; 230 V from any item to any other item is has the same potential outcome as 400 V - ie death by electric shock.
So not logical to worry about the voltage level being 400 V ; unless all 230V scenarios that are at least as likely have been dealt with.
Looking at likelihood; it's much more likely that there would be "2 faults" than "4 faults".
Eg a single item with both an earth fault and an open-circuit PEC; which would result in 230 V to any other object at earth potential.
And there's a high chance of another earthed item being in the vicinity.
So it's not logical to think that 4-fault scenarios will have been covered unless all 2- or 3-fault scenarios have been covered.
In fact there are a huge number of scenarios that are equally dangerous as, but much more likely to occur than; the one proposed.
And we don't have general rules in place for those.
Our Wiring Rules are based on IEC Standards.
Internationally, the acceptable level of safety for "fault protection" is accepted as being designed and installed to be safe under single-fault conditions.
Our rules allow for four methods of providing fault protection [1.5.5]; of which the most common - and the one in these scenarios -is automatic disconnection of supply in event of a fault. For this we use an earthing system in combination with a protective device.
For general cases the protective device is an overcurrent device, thus requiring a high-integrity earthing system.
This combination removes the danger in a short time; thus reducing the chances of anyone actually receiving a shock.
For situations deemed to be higher risk, and therefore justifying "additional protection"; we insist on the device being an RCD.
This provides even faster operating time for first (earth) fault; plus some mitigation-of-effects even for a second fault such as a broken PEC.
That's where NZ has set the acceptable level of safety.
Individuals are free to do more if they want to; but the increased cost of doing more isn't imposed by Rules.
For example, an RCD on either circuit would reduce the duration of shock current to a sub-lethal value (for most people) in the 4-fault scenario.
But Wiring Rules don't force people to have RCDs for sockets in offices ; because the stats say the cost of such a rule are not justified.
- open-circuit PEC in item 1
- earth fault on item 2
- open-circuit PEC in item 2
That's not just 4 simultaneous faults; but a particular combination of faults.
Plus a fifth condition: that the two items are simultaneously accessible.
It's true this scenario could result in a hazardous voltage between two simultaneously accessible items;
and the suggestion was not just that there should be a rule to avoid this, but that there already is one.
Which demonstrates a lack of joined-up thinking.
The point of having a set of rules for installation wiring is to manage the risks of using electricity.
It's simply not possible to make it risk free; so we aim for an acceptable level of risk - at an acceptable cost.
(And in setting that level; the risks & costs of not using electricity for the task have to be considered as well).
Risk management 101 says that risk is a function of the likelihood of an event combined with effect if it were to happen.
Before making rules to avoid a very unlikely scenario; logically we'd have to have rules that eliminated all the more likely scenarios that could result in a similar outcome.
Looking at outcomes; 230 V from any item to any other item is has the same potential outcome as 400 V - ie death by electric shock.
So not logical to worry about the voltage level being 400 V ; unless all 230V scenarios that are at least as likely have been dealt with.
Looking at likelihood; it's much more likely that there would be "2 faults" than "4 faults".
Eg a single item with both an earth fault and an open-circuit PEC; which would result in 230 V to any other object at earth potential.
And there's a high chance of another earthed item being in the vicinity.
So it's not logical to think that 4-fault scenarios will have been covered unless all 2- or 3-fault scenarios have been covered.
In fact there are a huge number of scenarios that are equally dangerous as, but much more likely to occur than; the one proposed.
And we don't have general rules in place for those.
Our Wiring Rules are based on IEC Standards.
Internationally, the acceptable level of safety for "fault protection" is accepted as being designed and installed to be safe under single-fault conditions.
Our rules allow for four methods of providing fault protection [1.5.5]; of which the most common - and the one in these scenarios -is automatic disconnection of supply in event of a fault. For this we use an earthing system in combination with a protective device.
For general cases the protective device is an overcurrent device, thus requiring a high-integrity earthing system.
This combination removes the danger in a short time; thus reducing the chances of anyone actually receiving a shock.
For situations deemed to be higher risk, and therefore justifying "additional protection"; we insist on the device being an RCD.
This provides even faster operating time for first (earth) fault; plus some mitigation-of-effects even for a second fault such as a broken PEC.
That's where NZ has set the acceptable level of safety.
Individuals are free to do more if they want to; but the increased cost of doing more isn't imposed by Rules.
For example, an RCD on either circuit would reduce the duration of shock current to a sub-lethal value (for most people) in the 4-fault scenario.
But Wiring Rules don't force people to have RCDs for sockets in offices ; because the stats say the cost of such a rule are not justified.