eSP Outdoor Mounted Slip-Over Bushing Current Transformer


The Problem:
You have quite a few of the SP/SPS circuit breakers in your fleet and you have noticed that some of the CTs on the older models have cracked casings. Cracks and fissures in the CT casings allow for moisture ingress, which will diminish the overall performance (reduced insulation performance) and produce unacceptable insulation resistance values. While some have tried caulk, sealants and glues, these do not hold up and prevent leaks for long and they do not resolve the real issue – deteriorated insulation resistance values. Other than the cracked casing issue, the SP/SPS breaker has been a pretty good breaker. Since you have quite a few of them and the cost to replace the entire breaker (including all of the civil work) is not cheap, you really don’t want to spend your capital money on replacing the entire breaker. You could replace the CT with another one that is just like the OEM model, but do you really want to replace the failing CT with one constructed using the same technology? Especially when you can replace it with a design that eliminates the cracked casing issue, is made for the exact footprint, and gives you the capability to UPGRADE for a configuration that does not limit you to only 2 CTs per bushing in some configurations. What are you going to do?

59.9% of all tested SP/SPS CTs with cracked casings had insulation resistance value below 0.2 M-ohms and 89.9% of all tested CTs with cracked casings had insulation resistance value below the generally accepted MINIMUM of 1 M-ohms. 89.9%!

In addition, while you can see the cracks that are on the top of the upper CT’s casings and on the perimeter of all CT casings, you don’t know if the tops of the other CTs are cracked.

(The generally accepted MINIMUM insulation resistance value is 1 M-ohms, per IEEE 57.13.1 Guide for Field Testing of Relaying CTs.)

The Solution:
The solution is Meramec’s eSP Outdoor Mounted Slip-Over Bushing Current Transformer.

The toroidal core is continuously tape-wound using cold-rolled, grain-oriented electrical grade silicon steel, which receives a full stress relief anneal after it is wound to its specified dimensions. The secondary winding is then wound of insulated copper magnet wire over the insulated core with the turns equally spaced around the core periphery. When taps are pulled, they are wound in a manner that assures a fully distributed winding between any connections. The coil is then fully encapsulated in an ANSI 70 (sky grey) resin system that has been developed and tested for outdoor use. The potting compound is U.V. stabilized, is flexible to allow for expansions caused by temperature variations from -50°C to +65°C, has good chemical resistance, high dielectric strength, rugged physical properties to endure mechanical shock, and overall excellent weather resistance. The eSP is cast with aluminum mounting hubs which are used to transfer the mounting forces away from the potting compound, and are fixed to align with the existing bolt pattern.

The eSP is custom-designed to fit over the existing bushing (without removal) directly onto the interrupter frame or onto an existing CT if present. In most OEM installations, a maximum of only 2 CTs were mounted per pole. With this new design, in some instances a 3rd CT may be installed, provided the stack height does not exceed the bushing flange height. If replacing existing CTs, the same hardware can usually be used. If the new stack height exceeds the existing stack, or if adding an additional CT over an existing CT, then a hardware kit will be required. The top most CT of the stack shall be covered with a custom-fitted ground shield. What makes Meramec the right choice is our customization, quality and delivery. Every CT we produce is custom-designed to your specific application. Our engineering design, our technicians’ attention to detail, and our 100% verification all contribute to the achievement of our 5 year 99.978% quality performance rating. Our customer service is paramount as well. If you are in a scheduled maintenance mode, we deliver as needed. If you are in a downtime crisis situation, we pull out all the stops to get you up and running.

Additional Features & Options
  • Custom ratios and accuracies are available, in addition to those listed on the product sheet.
  • Can be designed to IEC and CSA standards.
  • Can be designed for 50Hz power frequencies.
  • Conduit box can be supplied with hub sizes up to 1.5” NPT.
  • In some cases, dual-core units may be possible.
  • In some instances, 3 units can be installed on a pole.

The Meramec Difference:
  • We are available 24/7 to support you.
  • RFQs are quickly turned.
  • Delivery performance
    • In project mode, rest assured it is delivered on time.
    • In crisis mode, we pull out all the stops to get you up and running.
  • Custom CT designs with custom mounting.
  • Known and proven product quality – install and forget.
  • Technical support that allows your engineers to focus on other issues and not the CT’s design.
  • 99.978% Quality Performance.
  • We are the Instrument CT Specialists. Instrument Transformers are what we do.




Meramec’s Rapid Response is available for rush situations. We are available to support your needs. Designs can be made to match the existing current transformer characteristics or to optimize your chosen parameters. Our products are guaranteed and backed by our 99.978% quality performance rating.

When you are in need of Current Transformers to directly replace existing units, to upgrade your system, or to outfit new equipment – Spec Meramec…When it has to be Right!