Cable through shielded enclo­sure wall

Shielded con­tainer can be suc­ces­sively used as pro­tec­tion against elec­tro­mag­netic fields, elec­tro­mag­netic pulses cre­ated by light­ning. But one of the most crit­i­cal part, beside doors and ven­ti­la­tion aper­tures, is cabling to pro­vide power and com­mu­ni­ca­tions inside con­tainer.

There is con­tainer Fig.1. (Fara­day cage) which is used to pro­tect sen­si­tive equip­ment from ambi­ent fields, elec­tro­mag­netic pulses and light­ning direct and indi­rect impact. As equip­ment inside needs AC power there should be cable that con­nects elec­tronic equip­ment from inside to out­side. To achieve it is nec­es­sary to drill a hole in 3mm thick steel wall and bring the cable through.

In shield­ing indus­try this sit­u­a­tion is usu­ally solved by using feedthrough fil­ter at the cable entry. In this way cable is fil­tered and no fields should pen­e­trate inside the con­tainer (depends on feed through fil­ter parameters).

Fig. 1. Con­tainer used for equip­ment pro­tec­tion in harsh elec­tro­mag­netic envi­ron­ment

Such fil­ters are quite expen­sive and they are not an ordi­nary prod­uct in elec­tronic com­po­nent shop. Is there other tech­niques to solve this issue with­out feedthrough fil­ter usage. There are some tech­niques to try:
1. Use cable with­out shield­ing (ref­er­ence case).
2. Use shielded cable and leave shield­ing uncon­nected (ref­er­ence case as exam­ple of poor EMC/​EMI knowl­edge dur­ing instal­la­tion process).
3. Use shielded cable and con­nect shield­ing at one end only (ref­er­ence case as exam­ple of poor EMC/​EMI knowl­edge dur­ing instal­la­tion process)
4. Use shielded cable and ter­mi­nate shield­ing at both ends.
5. Use shielded cable and ter­mi­nate shield­ing at both ends, and place it in metal­lic pipe.

To eval­u­ate these tech­niques 3D mod­el­ling is used. Sim­ple con­tainer model is rep­re­sented in Fig. 2. Con­tainer has cable entry in 40mm diam­e­ter. Dimen­sions of con­tainer for harsh elec­tro­mag­netic envi­ron­ment 3m x 2,35m x 2,39m.

Fig.2 Con­tainer model with 40mm aper­ture in wall used for harsh elec­tro­mag­netic envi­ron­ment

To pro­vide power inside con­tainer, cable con­sist­ing of two con­duc­tors is used. Cable con­duc­tor cross sec­tion is rec­tan­gu­lar, to speed up mod­el­ling. It has no impact on mod­el­ling results. Cable is lied out as can be seen in Fig.2a. and Fig. 3. Out­side the con­tainer cable is routed only in par­al­lel plane to the con­tainer wall. Inside the con­tainer cable is routed straight along the con­tainer as typ­i­cal over­head instal­la­tion.

Fig. 2a Con­tainer model with two wire cable

First case: Cable pen­e­trates con­tainer wall with­out any EMC/​EMI protection/​sealing. Con­tainer cut and cable entry zoom is given in Fig.3.

Fig. 3. Con­tainer model cut. Two wire cable entry zoomed.

Sec­ond case: Shielded cable pen­e­trates con­tainer wall. Cable shield­ing is not con­nected on both sides. Con­tainer cut and cable entry zoom is given in Fig.4. This case will sim­u­late sit­u­a­tion when shielded cable is used for EMC/​EMI protection/​sealing, but for some rea­sons shield­ing is not con­nected (usu­ally it is assem­bly error due to worker’s poor knowl­edge of EMC/​EMI pro­tec­tion).

Fig.4 Con­tainer model cut. Shielded two wire cable entry zoomed. Shield­ing not con­nected on both sides

Third case: Shielded cable pen­e­trates con­tainer wall. Cable shield­ing is con­nected only on the side where it pen­e­trates con­tainer wall. Con­tainer cut and cable entry zoom is given in Fig.5. This case will sim­u­late sit­u­a­tion when shielded cable is used for EMC/​EMI protection/​sealing, but cable is “360deg” ter­mi­nated only at the one side.

Fig.5 Con­tainer model cut. Shielded two wire cable entry zoomed. Shield­ing con­nected on cable entry in con­tainer.

Fourth case: Shielded cable pen­e­trates con­tainer wall. Cable shield­ing is con­nected on both sides using good “360deg” ter­mi­na­tion. Con­tainer cut and cable entry zoom is given in Fig.6. For cable con­nec­tion and shield­ing ter­mi­na­tion metal­lic con­nec­tion box is installed on con­tainer wall.

Fig.6 Con­tainer model cut. Shielded two wire cable entry zoomed. Shield­ing con­nected on both sides. Extra shielded con­nec­tion box added.

Fifth case: Shielded cable pen­e­trates con­tainer wall. Cable shield­ing is con­nected on both sides using good “360deg” ter­mi­na­tion. In addi­tion extra shielded tube is used on top of shielded cable between con­nec­tion box and con­tainer. Con­tainer cut and cable entry zoom is given in Fig.6. Extra pro­tec­tion usu­ally is used to pro­tect cable from mechan­i­cal dam­age, extra seal­ing for water and dust. It also pro­vides backup if cable shield­ing is not prop­erly ter­mi­nated due to worker poor knowl­edge or lack of skills.

Fig. 7. Extra shield­ing added on top of cable shield­ing.

Sixths case: Cable pen­e­trates con­tainer wall through feed through fil­ter capac­i­tor. Con­tainer cut is given in Fig.8. This is con­ven­tional way to pro­vide “clean” power inside con­tainer with­out RF noise.

Fig. 8. Feed through fil­ter on each line 4.7nF. No cable shield­ing used

Eval­u­a­tion of upper men­tioned tech­niques will be tested with hor­i­zon­tally polarised plane wave 100V/​m in mag­ni­tude. Elec­tro­mag­netic wave will be gen­er­ated at the side of cable entry in con­tainer as in Fig. 9. Field will be mon­i­tored inside the con­tainer in the geo­met­ri­cal cen­tre. Addi­tion­ally, field will be recorded in the con­tainer cut to visu­al­ize field inside the con­tainer that should pro­tect equip­ment from elec­tro­mag­netic envi­ron­ment. Fre­quency range is cho­sen 100MHz-​800MHz, but it could be extended in future research.

Fig. 9. Con­tainer exited by hor­i­zon­tally polarised plane wave from the left side

Fig. 10. Con­tainer model cut. Two wire cable entry. 800MHz E-​Field mod­el­ling

Fig.11 Con­tainer model cut. Shielded two wire cable entry. Shield­ing not con­nected on both sides. 800MHz E-​Field mod­el­ling

Fig.12 Con­tainer model cut. Shielded two wire cable entry. Shield­ing con­nected on cable entry in con­tainer. 800MHz E-​Field mod­el­ling.

Fig.13 Con­tainer model cut. Shielded two wire cable entry. Shield­ing con­nected on both sides. Extra shielded con­nec­tion box added. 800MHz E-​Field mod­el­ling.

Fig. 14. Extra shield­ing added on top of cable shield­ing. 800MHz E-​Field mod­el­ling.

Fig. 15. Feed through fil­ter on each line 4.7nF. No cable shield­ing used. 800MHz E-​Field mod­el­ling

Fig.16 Con­tainer model with 40mm aper­ture in wall used for harsh elec­tro­mag­netic envi­ron­ment. 800MHz E-​Field mod­el­ling


Sum­mary

In Fig. 17. field mea­sure­ments in fre­quency range 100MHz– 800MHz is sum­marised to eval­u­ate each case. It is clearly vis­i­ble that unshielded cable and shielded cable, where shield­ing is float­ing or con­nected at one end per­form sim­i­larly, offer­ing poor pro­tec­tion inside con­tainer. Fields as high as 100V/​m can be expected inside the cham­ber, the same amount as out­side. Respec­tively, at some fre­quen­cies there is no dif­fer­ence– there is shielded enclo­sure or there is open air.

Much bet­ter sit­u­a­tion is in case if cable shield­ing is ter­mi­nated at both ends and in case if cable shield­ing is ter­mi­nated at both ends and addi­tional shield­ing is added. Fields up to 6V/​m can be expected inside the cham­ber, while plane wave out­side the con­tainer pro­vides 100V/​m. Field strength 6V/​m should not be a prob­lem for equip­ment that is designed for indus­trial envi­ron­ment, where equip­ment is usu­ally tested with 10V/​m immu­nity level. But it can cre­ate prob­lems for sen­si­tive equip­ment.

If sus­cep­ti­ble equip­ment is used inside the cham­ber feedthrough fil­ter usage should be con­sid­ered. Feedthrough fil­ter for unshielded cable could pro­vide excel­lent pro­tec­tion from elec­tro­mag­netic fields. In this case field inside the con­tainer is lower than 0.2V. If shielded cable is used and shield­ing is ter­mi­nated at both ends, feedthrough fil­ter can give even bet­ter result– 0.04V/m. Shield­ing gives bet­ter per­for­mance in higher fre­quency range.

As ref­er­ence, sit­u­a­tion with­out cable is mod­elled. If there is only 40mm diam­e­ter aper­ture in con­tainer wall, fields inside con­tainer are below 0.2V/m. Small hole in con­tainer wall is not a prob­lem, the prob­lem is cable, that trans­mits fields from out­side inside.

Fig. 17. Field mea­sure­ments in con­tainer


Visu­al­iza­tion


Con­tainer model cut. Two wire cable entry. 800MHz E-​Field mod­el­ling.



Con­tainer model cut. Shielded two wire cable entry. Shield­ing not con­nected on both sides. 800MHz E-​Field mod­el­ling.



Con­tainer model cut. Shielded two wire cable entry. Shield­ing con­nected on cable entry in con­tainer. 800MHz E-​Field mod­el­ling.



Con­tainer model cut. Shielded two wire cable entry. Shield­ing con­nected on both sides. Extra shielded con­nec­tion box added. 800MHz E-​Field mod­el­ling.



Extra shield­ing added on top of cable shield­ing. 800MHz E-​Field mod­el­ling.



Feed through fil­ter on each line 4.7nF. No cable shield­ing used. 800MHz E-​Field mod­el­ling.