Cable Design Software

[timbobelfast]

Can anyone recommend a decent Cable Design Software package?

I have worked out a recent calculation on paper, now I would like to confirm my findings with software.

Its an existing run of 2X 120mm 3C Al SWA to feed a 185 Kw motor......….

My calcs tell me disconnection time in 5 seconds will not be met

 

[pc1966]

That is a big motor! Is it on 400V, guess it must be something like 380A running? That must be getting close to the limits for those cables so you want to be sure!

 

[pc1966]

Are the cables, etc, already in? If so it might be cheaper to use a MCCB RCD set to 1A or whatever trip threshold to achieve fast disconnection on an Earth fault than to replace them. You would still need to verify the cable is OK adiabatically for fault clearing without Earth.

 

[Spoon]

Is the motor being controlled by a drive?
We use motors of that size but use the cables from the motor or drive manufacturer.

 

[timbobelfast]

Is the motor being controlled by a drive?
We use motors of that size but use the cables from the motor or drive manufacturer.

No, its on an ABB Soft starter only. It's driving a fire pump.

 

[timbobelfast]

Are the cables, etc, already in? If so it might be cheaper to use a MCCB RCD set to 1A or whatever trip threshold to achieve fast disconnection on an Earth fault than to replace them. You would still need to verify the cable is OK adiabatically for fault clearing without Earth.

Cables already in, existing motor is 165 kw and has low Ir. Its on 400g500m bs88 fuses. Oddly enough, no semi conductors to protect the soft starter.

Adiabatic equ only gives me 239A. Fuse won't operate for about a week

 

[pc1966]

Adiabatic equ only gives me 239A. Fuse won't operate for about a week

Eh? But that is less than the running current!

What length and supply Ze do you have?

 

[timbobelfast]

120 m Zdb at panel is 0.05

 

[pc1966]

If I look at Table 4H4A and they are clipped direct it is 404A for 2 * 120mm 3-phase cables, and the cable impedance in Table 4H4B gives 31.8 + j 8.1 mOhm impedance for 120m (ignoring any inductive influcent between the cables).

Assuming the 0.05 is restive (possibly not true...) that give |Zs| = 82.2 mOhm and for 230V to earth PFC = 2798A, or 2658 at 0.95 Umin. Looking at the fuse curve here:

http://espm.co.uk/BS88%20fuse%20info.pdf

For 400M500 fuse it looks like it only just meets 5s disconnection time.

But someone out there ought to have a better way of checking it!

 

[pc1966]

Doh, forgot it probably is relying on the armour for CPC?

In that case R2 is around 0.76 mOhm/m so for 120m of cable (assuming similar to copper SWA) so computing again:

Z cable​	0.53​	j0.135​	mOhm/m​
R1​	0.265​	​	mOhm/m​
R2​	0.76​	​	mOhm/m​
R1+R2 per m​	1.025​	j0.135​	mOhm/m​
Length​	120​	​	m​
R1+R2​	123​	j16.2​	mOhm/m​
|R1+R2| (one cable)​	124.1​	​	mOhm​
|R1+R2| (parallel pair)​	62.0​	​	mOhm​
Ze​	50​	​	mOhm​
Zs​	112.0​	​	mOhm​
Umin​	218.5​	​	V​
PFC​	1950​	​	A​

Looking up the gM 400M500 fuse it appears to be around 12-15s disconnection time.
[automerge]1591654489[/automerge]
Armour from last page of this document:

https://www.whyprysmian.co.uk/BS5467.pdf

 

[pc1966]

Doh! Realised I used the figure for 4C armour! I guess that illustrates the point about good software to minimiser errors. Trying again with 1.2 mOhm/m for the armour as CPC:

Z cable​	0.53​	0.135​	mOhm/m​
R1​	0.265​	​	mOhm/m​
R2​	1.2​	​	mOhm/m​
R1+R2 per m​	1.465​	0.135​	mOhm/m​
Length​	120​	​	m​
R1+R2​	175.8​	16.2​	mOhm/m​
Z (one cable)​	176.5​	​	mOhm​
Z (parallel)​	88.3​	​	mOhm​
Ze​	50​	​	mOhm​
Zs​	138.3​	​	mOhm​
Umin​	218.5​	​	V​
PFC​	1580​	​	A​

Eyeballing the graph that is something like 30-60s fusing time.

The table on Fig 3A3(c) has 2580A for 5s on a 400A fuse, so Zs limit is 84 mOhm, given your Ze of 50 the cable needs to be below 34 mOhm which would mean an additional CPC of around 20 mOhm or less impedance, but that seems impractical given the sort of inductance present (e.g. Table 4H1B).

So I suspect the only practical solution given the cost & trouble of replacing the cables might be a MCCB style of RCD. Anyone other ideas?

 

[Alex-SL]

Hello,
If you were using a 4-wire cable, then I would advise the MeteorSpec software to choose and check cable cross-sections. The MeteorSpec has the motor calculator and the cable current-carrying capacity calculator. The freeware version supports only 3-phase symmetrical networks with 4- or 5-core power cables.

I made calculations of load current with power factor = 0.85 and motor efficiency = 95%.

To perform any electrotechnical calculations in a complex electrical circuit, you must first build its model. With the MeteorSpec you can calculate all parameters in low-voltage networks.

To calculate short-circuit currents at the consumer I made a simple preliminary model with one Cu-cable section 2x(4x120mm2) from a transformer to the motor. If the real network has a complex structure, we can freely simulate it if need.

This preliminary model is build having an imaginary data of the transformer.

According to the preliminary model the minimum SC-current (1-phase at 90C) is 6.8kA. Then, for 400G500M fuse the disconnection time is around 1sec. Of course, if the feeder from the transformer to the motor consists of several cable sections, the disconnection time will be longer.

If you can send me the all cable data (CSA, lengths) from transformer to the consumer and the transformer data, I can make an accurate model of the entire feeder and calculate the SC currents and voltage losses at each node and at the end of the feeder in operating and starting modes.

 

[Andy-1960]

You might like to look at the regulations regarding fire pumps and their associated protection, it generally differs from what we are used to. It does vary depending on insurance companies and local fire officers, but generally this is what is done.

1. They are usually connected to the live side of the incoming supply, so when everything else in the building is off, the supply to the fire pump is still on.
2. Usual thermal / magnetic trips are not used with fuses the norm for protection, thermal overloads are not usually fitted.
3. Fuses are rated to carry the locked rotor current for 75% of the time calculated for winding failure.

I think the reasoning behind it is, if the building is on fire, what’s the point in saving the motor!

 

[timbobelfast]

You might like to look at the regulations regarding fire pumps and their associated protection, it generally differs from what we are used to. It does vary depending on insurance companies and local fire officers, but generally this is what is done.

1. They are usually connected to the live side of the incoming supply, so when everything else in the building is off, the supply to the fire pump is still on.
2. Usual thermal / magnetic trips are not used with fuses the norm for protection, thermal overloads are not usually fitted.
3. Fuses are rated to carry the locked rotor current for 75% of the time calculated for winding failure.

I think the reasoning behind it is, if the building is on fire, what’s the point in saving the motor!

Do you have the British standard available to cover fire pumps?

The pump is on a jetty.

There is redundancy on the site by means two new diesel power fire pumps.