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Splicing Shielded MV Cable to Non-Shielded MV Cable

Splicing Shielded MV Cable to Non-Shielded MV Cable

(OP)
I have a situation where we need to splice a length of shielded 8kV rated cable to non-shielded 5 kV cable for a failure repair on a 4160 volt system. Does anyone know if the 8kv cable can be treated similarly to the unshielded cable within the splice or will excessive voltage stresses be present without properly terminating the shielded cable conductors? The cable and splice kit manufacturers do not have information available to address this situation.  

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

If this is anything but a very temporary fix, you need to properly terminate the shielded cable. Otherwise it will fail as you say, because of the high voltage stress at the end of the shield. Also, a proper termination is needed to seal out moisture. Then you can connect to the unshielded cable.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

You definitely need the correct pothead and other temination devices at the end of the shielded cable - you then continue the short distance with unshielded cable.

Is there any reason for using 8 KV cable on 4,160 volts? 5 KV 3 conductor metal clad cable that has only the sheath as an overall shield works quite fine on 4,800 volts ungrounded.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

(OP)
Existing cable is 600 MCM. Local electrical inspector has demanded that ampacity of spliced section be the same as existing cable. 600 MCM is only immediately available as 8 kV shielded.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

Can you make a temporary fix until you can obtain the matching unshielded cable?

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

Why don't you use 2 conductors in parallel for each phase? This could use more readily available and compatible unshielded cable. You will need 2 conduits for the short section.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

In addition to the issues already mentioned, you need to remember this will present some safety related issues also.  Without proper grounding of the 8kV cable shield, it will act as a capacitor and maintain a dangagerous voltage even when the cable is de-energized.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

(OP)
Cable is Teck 90 armoured so integrity of armour sheath wold be maintained over splice location eliminating exposure to voltage on unterminated conductor shield.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

If the shield is not grounded, the inner jacket will have to withstand some significant portion of the line-to-ground voltage, for which it would not typically be designed.

Also, most codes require the shield to be grounded regardless of presence of armour jacket.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

(OP)
The Canadian Electrical Code only requires the shield to be grounded in one location as a minimum which it would be at the cable termination.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

Just for your info 3M's 5550 Series splice kits address shielded to non shielded connections right in their installation instructions.

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

Note that under the 2005 NEC, section 310.6 requires that all insulated conductors operated above 2000 volts, be of the shielded type.
Don

RE: Splicing Shielded MV Cable to Non-Shielded MV Cable

(OP)
Shielding isn't required for cables up to 5kV under the Canadian Electrical Code if it is permanent and installed in totally enclosed dry metal raceway or is metal armored cable.

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