Difficult overload omissions question

[rich3009]

Q12 Omission for circuit protection against overload may not be given to
which of the following:
1. Fire Panel supplies
2. Control circuit for fire extinguishing equipment
3. Exciter circuits for rotating machines
4.Supply circuits for lifting magnets

I'm guessing it's the control circuit but struggling to find the definitive answer in the regs, 433.3.3 doesn't clarify it absolutely

 

[telectrix]

my thoughts would be #3, as #1, #2, and #4 are IMO fixed loads. might be wrong though.

 

[Deleted member 26818]

Out of the four circuits, which one would not present a danger if it were de-energised?

 

[rich3009]

According to regs no 3 would present a danger if de-energised, plus the question asks which circuit is not allowed to have overload protection omitted, exciter circuits are clearly defined as being ok for Omission in 433.3.3

 

[rich3009]

As would the others

 

[rich3009]

I'm edging towards fire Panel supplies as they have battery backup and warnings of power failure

 

[Spoon]

Is it a trick question as they all can have protection omitted...

433.3.3
The omission of devices for protection against overload is permitted for circuits supplying current-using equipment where unexpected disconnection of the circuit could cause danger or damage.

Examples of such circuits are:

(i) the exciter circuit of a rotating machine = (3. Exciter circuits for rotating machines)

(ii) the supply circuit of a lifting magnet = (4.Supply circuits for lifting magnets)

(iii) the secondary circuit of a current transformer

(iv) a circuit supplying a fire extinguishing device = (2. Control circuit for fire extinguishing equipment)

(v) a circuit supplying a safety service, such as a fire alarm or a gas alarm = (1. Fire Panel supplies)

(vi) a circuit supplying medical equipment used for life support in specific medical locations where an IT system is incorporated.

 

[Darkwood]

There are 2 regulations to consider here, omission for fixed loads and omission due to inherent safety issues of cutting power during an overload issue.
Electromagnets carrying heavy loads can be omitted as losing power would drop the load in an uncontrolled manner.
Fire panels and extinguisher controls could be defined under the fixed loads clause thus be allowed omission as modern circuits usually have back ups.
Exciter circuit on the other hand are generally used to start up motors and like any motor there must be some form of protection from windings overheating, stall etc etc although there are numerous methods to identify this I cannot see how one could be included in an omission unless there was a risk to safety in doing so !...

I too am not entirely sure on the answer but that's my input on this, it would depend what the tutor is looking for here, what stage of learning the pupil is at as to the answer(s) they may want, if omission through fixed load scenario hasn't been covered then it may not be represented in the answers here until it has at a later stage in the teaching process.

 

[Bobster]

Answers #3

 

[Darkwood]

Yep after reading spoons post it seems they all are but that really is subject to how the control system functions and what are the risks of power loss on an individual scenario, although it does ask what circuits as a general point and not a specific install example.

 

[rich3009]

Very poor exam question

 

[Deleted member 26818]

Should I be replacing the 13A fuses with nails on my drills, grinders and belt sanders?

 

[Darkwood]

The clause of ommission allows most household circuits not to have overload protection with the exception of sockets as most of the other circuits can be classed as fixed loads, although we tend to provide it as a rule of thumb to meet ELI requirements but the mcb rating does not always have to be lower than the ccc of the cable using the omission clause ... to be noted here we still need to provide short circuit protection to satisfy BS7671.

 

[Risteard]

Surely it's the control circuit as that isn't a circuit supplying the load.

 

[rich3009]

That was my logic

 

[Darkwood]

The BS60204 cover machinery control which is where some of this would sit regarding the magnet and the exciter circuit although it does say supply circuits to lifting magnet, it is a poorly worded question given omission of the overload can be done for other reasons too and anyone delving in these particular examples shouldn't be doing so without been converse in other British Standards so I find it a strange question to ask, the latter 2 rely on a person recognising how a control system operates and the safety features within it before establishing a risk assessment for an omission ... well beyond BS7671 IMHO, it borders on giving pupils a false sense of security if they indeed do come across said circuits with lack of O/L protection as they may consider it OK when in fact it is incorrect or missing given the particular design. I think the latter examples of a magnet and exciter circuit should remain in the relevant regulations and not cross over into the BS7671.

 

[rich3009]

To be honest when you get to this level of design, it's normally done by an electrical engineer not a sparky