Discuss MCB operation & cable length in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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I'm currently just at the start of my electrical training so please excuse what may be a dumb question. It's a few weeks till I'm back at college and so not able to ask the question there.

I want to understand a bit more about cable length. If you're using an MCB as overcurrent and fault protection then cable sizing, length etc clearly have an effect on Zs and therefore you may not make the fault disconnection times. If you have a situation where you don't meet the Zs requirements for a circuit, say on a TT system, so you are relying on RCD protection for fault protection, I want to understand two things:

How does cable length affect the response time of the RCD, say 30ma in this scenario? As the cable gets longer does it affect the tripping time?
Does cable length affect the MCB/Overcurrent operation to protect against fire if the circuit is overloaded but there are no fault conditions present?

If anyone can explain this to me that woiuld be great!
 
Not really.

overcurrent device would be affected as the resistance increases with length and with increased resistance gives increased earth fault loop impedance, thus increased disconnection times.

an rcd measured an imbalance. So if more current is going out via line conductors that returning through neutral it trips.

the imbalance happens within the rcd even if the leakage or loss is elsewhere in circuit.

I say not really because just to say no would not be exactly true.

if a cable was really long then there may be a delay in the imbalance if an earth fault occurred at the furthest point. But I would guess it would be minimal.

when testing RCDs you can test anywhere on the circuit As it doesn’t really matter for the operation of device. I normally test in the board itself.
 
To add
when testing RCDs you can test anywhere on the circuit As it doesn’t really matter for the operation of device. I normally test in the board itself.
When testing RCDs it is best to do this at the board, with the RCD’s output connections disconnected. Otherwise influences on the attached wiring (earth leakage, eg) could skew your RCD test results.
 
Welcome to the forum mate.
If you need access to the trainee forum please follow this link.
 
Welcome to the forum. And you seem to have a good grasp of the subject already.
To answer your second question:
Does cable length affect the MCB/Overcurrent operation to protect against fire if the circuit is overloaded but there are no fault conditions present?
Yes.
So when designing the circuit, care must be taken to ensure the cable length, and hence resistance, doesn't prevent the protective device from operating within the specified time for the circuit. In most cases, if the volt drop requirements are met, then so will the disconnection time be met.
 
Theoretically cable length could impact on RCD operation by reducing the fault current (as for MCB/fuse case), but in reality the current needed to trip an RCD is so small compared to the typical operating currents that you will simply never see that.

As @loz2754 points out, usually if you meet the volt drop requirement you will meet disconnection times. That comes from having no more than 5% drop, say, leading to a short circuit fault current L-N of 20 times the design current, and that leads to all common OCPD operating fast enough.

Where that rule-of-thumb fails is when you have a much higher earth impedance than neutral impedance, so while you might see x20 for L-N fault, you might have much less for the more dangerous (from a shock point of view) L-E fault.

For TN systems that is often at a fair proportion of the circuit length-limit you would have for volt drop anyway, and a quick look at the OSG Table 7.1(i) on pages 65-73 will answer the majority of design/installation problems where you can quickly check cable size, OCPD and method(s) to see if it is acceptable, and if so what length limit applies. (Generally I would alway desing for the "no RCD" limits of the OSG, and then the RCD is simply "additional" protection and not the primary means of meeting disconnection times)

But for a TT system this high Ze situation is the norm. There you are very unlikely to have an L-E fault current that would ever lead to OCPD disconnection, so an RCD is your only viable means of automatic disconnection in those cases.

Finally, RCD do not limit normal current flow L-N, so you also have to have some form of OCPD (or other means if limiting) and for most situations than means a RCD feeding a couple of MCB, where the functions are clearly separated, or the use of RCBO that combine both, which are the neatest solution if at all affordable.
 
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Does cable length affect the MCB/Overcurrent operation to protect against fire if the circuit is overloaded but there are no fault conditions present?
Generally speaking you have two classes of circuit protection:
  • Fault, where very large current flow
  • Overload, where a modest excess current flows
In the first case the cable length, or more specifically the resulting fault PFC/PSCC, determines how quickly the OCPD operates and so what sort of fault let-through energy is present. This let-through is the 'I2t' value that you can use to compute the minimum conductor size that will survive the fault. (Current squared gives heating per unit resistance, and 't' gives total heat energy from power x time)

You have to meet this fault protection requirement in practically every case, but it can result in OCPD that is well above the cable's current-carrying capacity. This is acceptable for fixed loads that you know are unlikely to overload, they either work, or they fault to trip things.

The second case is most common and applies to any circuit that has an unknown load, for example 13A sockets, 16/32A industrial sockets, lights that the user can change the lamp, etc. Here you have to chose the OCPD to match the cable's overload characteristics. Generally speaking, cable length has no impact on this overload protection, only the cable type, installation "method", and any heating from other cables, etc (i.e. the thermal environment that determines how hot it gets for a given current flow), but as you normally have to provide fault protection as well, length still matters.

Both fuses and MCB can provide both, but the distinction between them is most obvious for the MCB where:
  • Overload protection is provided by the thermal trip that offers roughly an inverse-time characteristic that sort-of mimics a cable's overload tolerance
  • Fault protection is provided by the magnetic trip, as that delivers disconnection well under 0.4s for limiting both shock risk and controlling the I2t
 
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