I'm new to the industrial world and was working with VSD cable the other day and as most of you will know, along with the screen there were 3 core and rather than a single earth there was 3
I understand VSDs cause harmonics or odd frequencies (I think) so I assumed it was something to do with this
Can someone explain to me why there are 3 separate earth's as opposed to 1 in most cables?
VSD Cable - why multiple earths?
Re: VSD Cable - why multiple earths?
My understanding - though I'm by no means an expert - is that there is no particular technical reason for using the 3-PEC config.
Not all VFD cables use this config, some have a single PEC. Splitting the PEC into thee smaller conductors allows then to sit in between the three actives for an overall smaller diameter cable with minimal need for fillers.
What's important is that the PEC (overall) be full size, ie at least as big in total CSA as the Actives (not smaller, as is usual for TPS).
It also has to be low-impedance at (very) high frequency, so needs to be fine-stranded to provide plenty of surface area.
When dealing with this stuff, it helps to use VFD instead of VSD; because while the end-user cares about the speed of the motor on the end, all the technical stuff relates to the frequencies involved. Not just the nominal frequency; but also - in fact mostly - the frequencies generated by the very fast switching of the electronics that generate the nominal frequency.
Not all VFD cables use this config, some have a single PEC. Splitting the PEC into thee smaller conductors allows then to sit in between the three actives for an overall smaller diameter cable with minimal need for fillers.
What's important is that the PEC (overall) be full size, ie at least as big in total CSA as the Actives (not smaller, as is usual for TPS).
It also has to be low-impedance at (very) high frequency, so needs to be fine-stranded to provide plenty of surface area.
When dealing with this stuff, it helps to use VFD instead of VSD; because while the end-user cares about the speed of the motor on the end, all the technical stuff relates to the frequencies involved. Not just the nominal frequency; but also - in fact mostly - the frequencies generated by the very fast switching of the electronics that generate the nominal frequency.
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Re: VSD Cable - why multiple earths?
And how does this comply for conductors in parallel if they are under 4mm²?
As I've said previously, I don't believe that the 4mm² rule applies for conductors within the same multi core.
Because of the wording in the clause: "parallel multi-core cables or groups of single-core cables " doesn't include conductors within one multi core cable.
As I've said previously, I don't believe that the 4mm² rule applies for conductors within the same multi core.
Because of the wording in the clause: "parallel multi-core cables or groups of single-core cables " doesn't include conductors within one multi core cable.
Re: VSD Cable - why multiple earths?
At risk of getting off-topic.
Not certain that 3.4.3 was intended to apply to PECs; because the clause is about the CCC of circuits; which only really affects Active(s) & Neutral.
Though, noting that 3.4.1 says "every conductor", PECs will have a required CCC so 3.4.3 logically should apply.
3.6.3 certainly doesn't apply, because that's about volt drop.
You may be correct that 3.4.3 doesn't apply to parallel cores within a single multicore cable, but there's no reason it shouldn't other than that the words don't specifically include this option.
The properties that lead to the rule being imposed (eg impedance, & physical robustness) are exactly the same regardless of whether the cores concerned are within the same cable or in different cables. So if the question were to be formally asked, I'm betting the answer would come in the form of new words to specifically include cores within same cable.
In fact a closely related question has been asked recently; WRT cables for lifts - where the standard cables supplied for this job generally have 2.5 mm2 cores, but with multi-storey buildings have issues with v-Drop to items in the lift car.As a result a special Exception for those is being proposed (based on them being a special type of cable). A general Exception for all flexibles was rejected; as was dropping the restriction altogether.
Which indicates that the relevant Committee believes the rule does apply to cores within same cable.
Not certain that 3.4.3 was intended to apply to PECs; because the clause is about the CCC of circuits; which only really affects Active(s) & Neutral.
Though, noting that 3.4.1 says "every conductor", PECs will have a required CCC so 3.4.3 logically should apply.
3.6.3 certainly doesn't apply, because that's about volt drop.
You may be correct that 3.4.3 doesn't apply to parallel cores within a single multicore cable, but there's no reason it shouldn't other than that the words don't specifically include this option.
The properties that lead to the rule being imposed (eg impedance, & physical robustness) are exactly the same regardless of whether the cores concerned are within the same cable or in different cables. So if the question were to be formally asked, I'm betting the answer would come in the form of new words to specifically include cores within same cable.
In fact a closely related question has been asked recently; WRT cables for lifts - where the standard cables supplied for this job generally have 2.5 mm2 cores, but with multi-storey buildings have issues with v-Drop to items in the lift car.As a result a special Exception for those is being proposed (based on them being a special type of cable). A general Exception for all flexibles was rejected; as was dropping the restriction altogether.
Which indicates that the relevant Committee believes the rule does apply to cores within same cable.
Re: VSD Cable - why multiple earths?
In a symmetrical earth design cable the earth conductors are located in the interces between the
phase conductors. This ensures the distance between the phase conductors and earth is consistent.
The resultant earth currents now cancel each other out minimizing the common mode current that
can flow between motor and drive.
On four conductor cable, the current flowing in the earth conductor is due to the
differing voltages induced on the earth conductor, caused by the distance from each phase
conductor to the earth conductor. This distance directly affects the capacitance between the
different phase and earth conductors.
Photos in attachment
phase conductors. This ensures the distance between the phase conductors and earth is consistent.
The resultant earth currents now cancel each other out minimizing the common mode current that
can flow between motor and drive.
On four conductor cable, the current flowing in the earth conductor is due to the
differing voltages induced on the earth conductor, caused by the distance from each phase
conductor to the earth conductor. This distance directly affects the capacitance between the
different phase and earth conductors.
Photos in attachment
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