Shield Strength of XLPE Cable

It comes as little surprise to find that individuals are so busy trying to find things using the wrong tools that the information needed remains obscured.

Rather than a mouse and a keyboard and an internet connection, it is far simpler to pick up the phone and actually talk to a human being at the cable manufacturer's premises. The telephone is the correct tool to use for such queries. Such interactions are personal, 2-way, and can yield information far quicker than an afternoon of searching on the internet ever can, with confirmation of understanding being achieved at the end of the call. And "time is money" when there is a problem to be solved.

CR4 does not offer training courses on the use of the telephone despite, according to a significant percentage of posts coming through, a significant need being evident.

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What is the shield material, method of fabrication and thickness. From this you can calculate the steady state current capacity.

For transients = a little more complex...

What is the shield for (lightning, RF, etc)

Who made the cable? Ask them the shields specs.

<...calculate the steady state current capacity...> The length, cable construction detail and method of installation are also factors according to protocols to be found in BS7671.

<...Who made the cable? Ask them the shields specs....> Quite. Over the telephone is the quickest way and highly recommended.

From the context, it appears to me that OP is the OEM of this cable. Can he not measure the resistance per foot with a DVM? Does he need to use a megger? 2000 amps at what potential (in case of irregularity in the shielding impedance)?

There are so many cable types and brands, I should think it really difficult to give any finite answer based on information presented thus far.

This seems like such an odd ball question. What is the questioner planning to do with/to the cable? Why are they planning that? Who are they doing that for?

So many questions, so few answers.

If they are planning to use this cable as a zip line, I would recommend against this, especially if their soldiers weigh more than 50 Kg.

You first need the CSA and the material of the shield, from that you can calculate its resistance. Using the adiabatic equation you calculate the temperature rise.

Failing that look it up in tables, larger cable sizes invariably fail.

That anyone would want to send 2000A down the shield of a cable for any longer than it takes the circuit protective device(s) to disconnect it is a thing of great mystery.

Sorry, Randall, I am afraid Lyn is right.

It is not about the ampacity [current carrying capacity] of the cable-500 mm^2 copper/XLPE in trefoil in air at 30oC ambient [900 A], but about short-circuit withstand current [about 70 kA rms for 1 sec.500 mm^2 copper conductor XLPE insulated] and mainly for the shield short-circuit withstand current. According to IEC 60502-2 recommendation for 400 mm^2 [no more than 400 mm^2 is tabulated in Table B1] 35 mm^2 it is the recommended. For 1 second this will withstand 5 kA.[250oC maximum from 90 rated].

However all this are suppositions only. Actually we don't know almost nothing.

..."Actually we don't know almost nothing".

Apparently, that means that we do know nearly everything...

I agree that pulsed current capacity would be way higher due to thermal dissipation limit.

How much amperage would they bad boys inject into these two ring grid neutrals before they trip the entire system on fault current?

To quote Rummy:

There known knowns; there are things we know we know.

We also know there are known unknowns; that is to say we know there are some things we do not know.

But there are also unknown unknowns - the ones we don't know we don't know.

And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.

I'm sorry, but in my opinion it is a childish jargon:

"strength" usually it is referred to a force-electric field intensity E or D, or magnetic field intensity or density [H or B] or even current density J but not a current since no live force it is in here.

Let's say-in translation-the maximum current the shield could withstand-rms[?],peak[?]how long,[6 cycles,1 sec,3 sec[?][ 500 mm^2] copper, aluminum[?] single-core, three cores, bunched, triplexed [?]

What type of sheath/shield:

[See-for instance:

ICEA[INSULATED CABLE ENGINEERS ASSOCIATION , INC.] P-45-482-1994

SHORT CIRCUIT CHARACTERISTICS OF METALLIC SHEATHS AND SHIELDS ON INSULATED CABLE]

1-wire-helically,braid or serving, longitudinally with corrugations

2-Helically applied tape ,not overlapped.

3-Helically applied flat tape ,overlapped.[new cable, good contact ]

4-Corrugated tape, longitudinally applied.

5-Tubular sheath

Io^2*t/A^2=247E-6*SG*SH*[(To+l)/ro]*Log10(T2+l)/(T1+l))

where:

Io=Short-circuit current ,amperes

A=Effective cross-sectional area of the shield or sheath, circular mils

t=Time of short-circuit ,seconds

SG=Specific gravity of shield or sheath material

SH=Specific heat of shield/sheath

To=Arbitrary temperature [20oC]

ro=Specific resistivity of shield/sheath material

l=Inferred temperature of zero resistance for shield/sheath material oC

T2=Maximum allowable shield or sheath transient temperature oC

T1=Operating shield or sheath temperature oC

I am constantly confused as to how some people get into the positions they hold.

1. If you don't know how to reply to this, you should.

2. Only the author of the statement knows what he really meant.

3. Asking total strangers to render a technical opinion based on no useable information is a waste of your client's time AND money.

4. You may not be qualified for your job.

5. Contact the author of the statement for clarification, or you may give them an incorrect answer to the question you thought they asked.

Contact the author of the statement for clarification, or you may give them an incorrect answer to the question you thought they asked.

you keep commenting like this?

if you don't understand what a questioner is asking, there are two options;

either ask for clarity or keep away from that post.

simple.

this is very rude of you. may be too much pride you have.

You're the one who asked the incoherent question.

I suggested that you ask for clarity, since you didn't know the answer.

This sounds like a problem with translation, and the requirement is not properly stated....I would think they are asking about the ability of the shielding to withstand 2000v megger for testing their field connections....but there will need to be clarification and you may need a proper translator.....

The key to answering this is the intended purpose the person posing the original query has in mind: "I want to interconnect the earthing of two ring main units."

Would not the answer to this depend very much on how these ring mains are earthed (grounded) presently? Perhaps the "user" wants special cable made up with this specification?

Are the attempting to inject such a current and disrupt the grid? Weaponize the grid? OMG! Call the NSA!!!

"Strength" is not a correct term for current-carrying capacity of a conductor, armor, or shield.

(500 mm) squared is a HUGE conductor. Correct terminology is 500 square mm, abbreviated as 500 mm².

Capable of carrying 2000 A for how long? Continuously, or until a fault is cleared by a protective device?

Maybe they want to know what the mechanical strength of the shield is while carrying 2,000AMPS.

I suggest you run a separate cable for earthing! That way you can install the correct csa of cable capable of carrying/handling a fault current of 2000A.

The other way is to calculate the actual fault current and run cable with either 4 of 5 conductors, one of those conductors being the earth conductor of the correct size!

Failing that, go find a reputable electrical engineer and pay him to sort it out for you!

LINK

Sorry. First of all, in my humble opinion, mrmuneb did not ask you to assist him but intended to interest YOU in something. It was for us benefit only, I think.

Second ,in my opinion, the cable about has to be medium voltage [6-36 kV] according to British Standard or IEC 60502-2. then ,in my opinion -I have to disappoint you-no 4 cores cable -standard- for medium voltage.

British Standard BS 7430 accepts cable shield as grounding conductor since in IEC World the shield cross section area is 4 times the UL recommended-in my opinion.

One does not need a grounding wire along the medium voltage cable.

Also, in my opinion,2000 A short-circuit current in a case of double ground fault it is very possible.

I made a mistake by not posting the complete information. sorry for that.

system voltage : 13,800 V

phase to ground voltage: 8,000 V

There are two feeders that are supplying power, but one of them is redundant and will kept standby.

we have laid single core 500 mm2 XLPE armored cables.

definitely, the armor shield has to be grounded from one side.

that is why the client might be interested to know the strength of armor regarding short circuit current.

Also the system is grounded via a 200 Amp, 40 Ohm grounding resistor.

And what are your earth fault ratings for the site?

Earth Fault Rating is: 200 Amp

"we have laid single core 500 mm2 XLPE armored cables.

definitely, the armor shield has to be grounded from one side."

Yes the armour HAS to be grounded, but NOT as an earth path/CPC, but to ensure the cable armour is not floating and in case of mechanical damage to the cable there is an earth.

Remember this is a difference between a CPC and earthing SWA!

"The use of the armour as the means of providing earthing to the equipment supplied by the cable (a function technically known as the circuit protective conductor or CPC) is a matter of debate within the electrical installation industry. It is sometimes the case that an additional core within the cable is specified as the CPC (for instance, instead of using a two core cable for line and neutral and the armouring as the CPC, a three core cable is used) or an external earth wire is run alongside the cable to serve as the CPC. Primary concerns are the relative conductivity of the armouring compared to the cores (which reduces as the cable size increases) and reliability issues." Wikipedia

In a single phase- to- ground fault: the grounding resistor[Ro=40 ohm] will limit the current to 200 A.

In phase-to-ground-to-phase short-circuit case [one at each end of the cable]

if the shield and armor are both ends grounded part of the short-circuit current could flow through shield/armor also.

If Z1=Z2 [far from generator] I"2k=1.05*sqrt(3)*13.8/|Z1+2*Zo|

Z1=Z1sys+Zxfm+Z1cabl Zo=Zocabl+(Rsh*RArmor)/(Rsh+RArmor)

In my opinion, if the cable length is more than 500 ft. the short-circuit current will not be more than 19 kA.

According to Nexans U.K.it could withstand this current.

http://www.nexans.co.uk/UK/files/Underground%20Power%20Cables%20Catalogue%2003-2010.pdf

Ish+armor=20.8 kA for 1*500 mm^2 copper cond. 15 kV XLPE insulated.

According to Prysmian U.K.

http://uk.prysmiangroup.com/en/business_markets/markets/ti/datasheets/1Core_15kV_cc_new.pdf

If the shield and armor are both ends grounded an induced current will flow through shield/armor always and this will produce an increased heating of cable and then a derating factor has to be applied. See IEC 60287-1-1/ 2.3.1.

In my opinion, in this case 62% derating[ampacity around 550 A at 90oC ].

If shield/armor is one end grounded only then an induced built-up voltage could be measured at non-grounded end. Usually up 25 V it is allowable.

There are possibilities to mitigate this voltage [See IEEE 575-cross bonding and sectionalizing ].